Why a house rule document?
So that players can know what rule changes might affect them up front. Player-facing rules go in this document, which gets given out to new players so they know what rules in the PHB don't apply. There are some DM-facing rules which don't go in this doc, including changes to monsters and procedures for running mysteries and dungeon crawls, and of course ad hoc rulings for a unique situation which don't get written down anywhere although I will try to be consistent if the unique situation recurs. This document exists so that players can make good decisions about things the characters would logically already know from experience.
So that players can know what rule changes might affect them up front. Player-facing rules go in this document, which gets given out to new players so they know what rules in the PHB don't apply. There are some DM-facing rules which don't go in this doc, including changes to monsters and procedures for running mysteries and dungeon crawls, and of course ad hoc rulings for a unique situation which don't get written down anywhere although I will try to be consistent if the unique situation recurs. This document exists so that players can make good decisions about things the characters would logically already know from experience.
House rules for my campaign
Simple changes
1.) On ability checks only, an odd score gives an extra +1. So Str 19 means you have +4 to Strength-based attacks and saves, but +5 to Strength checks.
2.) You can use both your move and your action in a Readied action, and can maintain a readied action from round to round.
3.) Class tweaks:
For Champion:
Improved Critical: you crit on a 19-20. Furthermore, when you inflict a critical hit, roll damage once and then double the total damage (including any bonuses from Strength/magic weapons/etc.), instead of just rolling twice the normal number of dice.
Furthermore, Remarkable Athlete now stacks with proficiency. So a Str 18 Champion 9 with Athletics proficiency would have +4+4+2=+10 to Strength (Athletics) checks, not just +8.
For Arcane Archer:
You have three shots per short rest instead of two.
For Battlemaster:
You can temporarily regain expended superiority dice, up to your normal maximum, by studying enemies for weaknesses. For every Attack you forgo during the Attack action, you regain one expended superiority die, which is usable only against creatures you can see at the time you regain the die. This temporary die expires after one minute if it has not already been used, as do any temp HP gained from Rally with it.
For Berserker:
When you end a Frenzy rage, if you pass a DC 15 Con save you do not suffer any exhaustion.
4.) Everyone uses spell points instead of spell slots. A player can opt for DMG spell points or use the rules here: https://bluishcertainty.blogspot.com/2016/12/spell-points-by-formula-5e-variant-rule.html. Must decide when spellcasting is first learned; cannot change.
5.) An attacker unseen by his target has advantage only on melee attack rolls, not ranged attack rolls; however, he does qualify for sneak attack damage at range if unseen despite not having advantage.
6.) Anyone with any weapon can attack vital areas at -5 to-hit for +5 to damage. GWM and Sharpshooter feats merely increase the bonus when you are using those weapons.
7.) Casting a non-bonus-action/non-reaction spell triggers an opportunity attack from any enemies in melee range, unless you have the Warcaster feat. (This replaces the third benefit of Warcaster, about reaction spellcasting.) This attack occurs after the spell is cast but before it takes effect (e.g. can still hit someone Dimension Dooring away, can disrupt a concentrations spell and prevent it from taking effect). If the attacker is a Mage Slayer, they can force a concentration save to potentially disrupt even non-concentration spells.
8.) Casting a non-bonus-action/non-reaction spell while moving at more than half speed, riding a horse or on a moving ship forces a concentration save every round even if it's not a concentration spell (Fireball) or it fizzles. Fizzling does not cost spell points but does waste your action to no effect.
9.) There is no Disengage. Opportunity attacks occur when you move at full speed away from an enemy (turning your back), or whenever you are paralyzed/unconscious. You can back away at half speed without turning your back. Creatures like beholders and black puddings have no backs to turn and can move at full speed in any direction without provoking opportunity attacks.
Remark: Dashing while moving backwards replaces and is equivalent to Disengage. You move half speed ('15), but you do it twice because you Dashed, so you move 30' without provoking opportunity attacks--that's why Disengage does not exist, because it's redundant.
10.) Falling damage doubles for every size category over Medium, and halves for every size category under Small. For example, an Ogre falling 100' would take 20d6 HP of damage, not 10d6, because it is Large; and a Fire Giant falling the same distance would take 40d6 damage because it is Huge; but a housecat would take only 5d6 because it is Tiny, and a rat would take 2d6 because it is Tiny II.
11.) Abilities which recharge on "rolling initiative" instead recharge after five minutes. Specifically the following:
I. Relentless [Battlemaster 15th IIRC]: five minutes after you expend your last superiority die, you regain one die.
II. Perfect Self [Monk 20]: whenever you've had less than four ki for five minutes and haven't spent ki during that time, you regain enough ki to have four ki remaining.
III. Superior Inspiration [Bard 20]: five minutes after you expend your last use of Bardic Inspiration, you regain one use of Bardic Inspiration.
12.) While you are incapacitated/stunned/paralyzed/unconscious (but not grappled/restrained), your Dex is 0. Won't affect PCs in heavy armor, but that swashbuckling rogue is in deep trouble if he ever gets paralyzed by a monster or put to sleep, even briefly.
13.) You do not heal to full health automatically on a long rest. Hit Dice can normally only be gained or spent on a long rest, instead of a short rest, and on any given rest you can spend HD or regain half of your HD but not both. However, Bardic Song of Healing now also allows you to spend one HD during a short rest.
14.) You can go below zero HP. Instead of the normal rules on death saves and stabilization, you die whenever you reach negative (max HP). E.g. if you have 40 max HP normally, you die at -40 HP. When you are at zero HP or below, you are either stunned or unconscious. (If you choose to make a DC 15 Con save and succeed you can be stunned, but if you fail the save or choose not to try, you are unconscious from shock.) When you are below zero HP and are not already stable, you must make a death save at the start of every round. If you succeed, you are stable unless/until you take damage again. If you fail, you take 20% of your max HP in damage, rounded UP, not down. You can be stabilized by another character's actions as usual, through the use of a healer's kit or the Wisdom (Medicine) skill or a Spare the Dying cantrip, and any amount of healing also stabilizes you, even 1 HP.
Example: if you have 40 HP normally, and you get hit twice by an Iron Golem for a total of 50 HP of damage, you're now at -10 HP (and likely unconscious, unless you made the DC 15 Con save). Since you're at -10 HP, not zero HP, you can't be restored to full activity by a simple 1 HP Word of Healing as you would under PHB rules--it takes 11 HP of healing to get you conscious again. At the start of every round, you make a death save (as usual, it is DC 10 and no attribute modifiers apply). If you succeed you stabilize at your current HP, otherwise you lose another 8 HP and must save again next round. If you ever reach -40 HP you die.
Remark: In some ways losing 20% of your HP is more generous than the default rules because it only takes one roll to stabilize, and someone who is just barely at negative HP may take five failures before they die. A wound which takes you down to -1 HP is extremely unlikely to kill you. In other ways though, it is less generous because stabilizing doesn't wipe out past failures--that requires actual healing. Furthermore, if you're deep in the negatives, a single failure will kill you, possibly before anyone else can intervene.
15.) Parry: This is a special type of attack which attacks attacks. When you Attack on your turn, you may choose to dedicate one or more of those attacks to Parrying. If an enemy attacks you with a melee weapon before your next turn, you may roll a melee weapon attack and replace your AC with your attack roll against that attack. You can do this a number of times equal to the number of attacks you dedicated to Parrying.
Example: Robilar the Mighty, an 11th level fighter, has been attacked in his bed by two assassins. Unarmed and unarmored, he snatches up a nearby log to use as an improvised club, and dedicates two of his three attacks to parrying. Robilar inflicts some damage on an assassin with his remaining attack, but then the assassins strike back. On the first assassin's attack, Robilar parries, and rolls d20+8 on his melee attack (for Strength 18 and proficiency bonus +4), getting a total of 23, which he uses instead of his normal unarmored AC of 10. The assassin rolls d20+6, gets a 15, and fails to hit AC 23! Then the second assassin strikes, and Robilar rolls d20+8 and gets a 14. The assassin rolls d20+6 and gets 17, so Robilar is hit! The assassin rolls 5d6+4 poison damage and inflicts 27 HP of damage on Robilar--Robilar is in trouble if he doesn't finish them off soon!
16.) To avoid breaking the game, Simulacrum works more like AD&D Simulacrum than PHB Simulacrum. Instead of an almost-perfect copy of the original, Simulacrum produces a dull, listless imitation of the original. If the original creature has any class levels or special abilities, the copy has only 50% of those class levels or abilities, rounded up (the player can select which ones, e.g. if you copy a dual-classed Fighter 5/Wizard 6, you can pick which feats to keep and if you want a Fighter 5/Wizard 1 or a Wizard 6).
In exchange for this nerf, Simulacrum is now not restricted to humanoids, and it may regain spell slots as normal by resting, but it never increases in power (never gains levels).
Complex changes
1.) Open-ended d20 rolls. Since skill checks and saves, unlike attack rolls, don't auto-succeed on a 20 or auto-fail on a 1, but I always want there to be some chance of failure*, on a 20 you re-roll at +10 and take the highest roll. Roll again at +20 if you roll another 20, etc. If you roll a 1, re-roll at -10 and take the lowest. If it's obvious that you've already failed or succeeded you can of course stop rolling already.
*Unless you have Reliable Talent.
2.) Concurrent multiclassing is an option. With concurrent multiclassing, you can advance in two classes or three at the same time, e.g. you could be a 10th level Battlemaster/Necromancer with the abilities of both a 10th level Battlemaster and a 10th level Necromancer. See https://bluishcertainty.blogspot.com/2017/01/5e-old-school-multiclassing-rules_29.html for more details.
3.) XP awards. All characters get a share of XP proportional to their share of the total levels or CR (rounded up to 1) on their side of a combat. For example, in a party of three 9th level PCs and one 5th level PC (total of 32 levels), if they earn 2000 XP from defeating twenty orcs, the 9th level PCs will all earn 9/32 * 2000 = 562.5 XP, while the 5th level PC earns 5/32 * 2000 = 312.5 XP. But if one of the PCs casts Animate Objects and temporarily animates 10 Tiny Objects during the fight, then there are 42 total levels/CR, so the 9thl level PCs earn only 9/42 * 2000 = 428.6 XP, while the 5th level PC earns 238.1 XP.
Purpose: this rule does not exist to punish you, it exists to keep the game interesting, so that you have a good excuse NOT to make the game too easy by flooding every fight with animated dead, purchased mastiffs, and summoned creatures unless you genuinely need them to survive and beat a tough enemy.
4.) Different initiative variant, WE-GO instead of IGO-UGO, and is designed to enhance player engagement and teamwork by reducing the amount of time players spend waiting for their turn to interact with the DM, while also making more intelligent characters and monsters seem more intelligent.
Procedure: DM secretly decides all monster actions while players consult each other and declare everyone's actions for the round together. Then actions are resolved in an order determined by the DM's best judgment of realism and convenience (e.g. arrows are faster than human feet so an arrow attack may happen before a move-and-melee-attack; but the DM might also resolve them both at the same time if the order isn't likely to change any outcomes), with initiative contests when the DM calls for one to decide potential ties.
Once you've declared an action or movement usage for this round you are committed and can't change it except how you initially specified (e.g. you can declare "I'm charging the goblins (moving towards them and Dashing if necessary) and attacking whoever gets within range if I didn't need to Dash"), but you can delay action declaration (or explicitly declare Delay). At any time before the round ends, you can declare an action or movement usage that you haven't used yet (e.g. "I'm standing up" after someone knocks you down, if you have enough movement left, or "I take cover") but then you automatically lose any initiative contests the DM calls for against those who declared actions before you. When the round ends, the DM will pause briefly for additional declarations, and if none are made (e.g. if a Mexican standoff occurs), any unused actions or movements are lost and a new round begins.
During initial action declaration for the round, faster thinking (a tighter OODA loop) is represented by letting highly intelligent creatures gain extra knowledge about other's actions before acting. A character (or monster) who wishes to observe other creatures before declaring an action may take a penalty of N on any initiative contests this round. If so, that character or monster may learn the action declarations of any characters or monsters with intelligence less than or equal to [character's own Int] - 10 + N, before declaring their own action. Example: if Erac the Mage (Int 17) does Observe 4, Erac's player may ask the DM what any monster with Int 11 or less is doing (which could be Delaying) before Erac has to declare his own action. If Erac chooses to Fireball because a group of goblins is preparing to scatter in all directions, and if the DM decides that an initiative contest is needed to see if the goblins scatter before the Fireball detonates, Erac will have -4 on that initiative contest because he paused to study the goblins before acting.
True surprise is rare and occurs only when an unwary target has effectively declared a non-combat action such as "read a book" at the same time a hidden attacker is preparing to attack them. If a target is wary (e.g. an adventurer in a dangerous dungeon) but unaware of a specific threat (the goblin aiming a crossbow at his back), at the start of combat the attacker will declare an action, and the target will be treated as having implicitly Delayed and will get to declare an action after the attacker's action is resolved.
Ask DM for details (or consult brief writeup at https://bluishcertainty.blogspot.com/2017/01/simultaneous-initiative-in-5e.html).
5.) Magic Resistance and Legendary Resistance works differently--requires a reaction and can dispel a spell it's affected by, regardless of whether or not it has a save. Details here: https://bluishcertainty.blogspot.com/2016/03/5e-magic-resistance-variant-rule.html Fundamentally, instead of advantage on saves, it's now like Dispel Magic as a reaction whenever a spell would directly or indirectly affect the monster.
If a creature attempts to use its magic resistance against a given spell and fails, that represents being unable to resist this casting of that spell unless its magic resistance improves--any retries will result in failure. E.g. if you've got a demon bound with Planar Binding, the demon gets only one chance to resist that Planar Binding. (But a crafty demon may not test the Planar Binding right away so be on your guard.)
If magic resistance fails due to temporary circumstances like Hex or Cutting Words, that represents a temporary failure which can be overcome if the creature retries without the hindance. In this rare circumstance, the DM may record the original d20 roll prior to the temporary modifiers, and re-use it on the subsequent attempts. (Or the DM may choose another equivalent method with the same probability curve.) Ditto for temporary improvements: a demon which rolls a 7 (failure) on its MR check against Planar Binding, but then receives Enhance Ability (Charisma) and tests the spell again, would roll one new die, compare it to the previous 7, and take the higher result.
Simple changes
1.) On ability checks only, an odd score gives an extra +1. So Str 19 means you have +4 to Strength-based attacks and saves, but +5 to Strength checks.
2.) You can use both your move and your action in a Readied action, and can maintain a readied action from round to round.
3.) Class tweaks:
For Champion:
Improved Critical: you crit on a 19-20. Furthermore, when you inflict a critical hit, roll damage once and then double the total damage (including any bonuses from Strength/magic weapons/etc.), instead of just rolling twice the normal number of dice.
Furthermore, Remarkable Athlete now stacks with proficiency. So a Str 18 Champion 9 with Athletics proficiency would have +4+4+2=+10 to Strength (Athletics) checks, not just +8.
For Arcane Archer:
You have three shots per short rest instead of two.
For Battlemaster:
You can temporarily regain expended superiority dice, up to your normal maximum, by studying enemies for weaknesses. For every Attack you forgo during the Attack action, you regain one expended superiority die, which is usable only against creatures you can see at the time you regain the die. This temporary die expires after one minute if it has not already been used, as do any temp HP gained from Rally with it.
For Berserker:
When you end a Frenzy rage, if you pass a DC 15 Con save you do not suffer any exhaustion.
4.) Everyone uses spell points instead of spell slots. A player can opt for DMG spell points or use the rules here: https://bluishcertainty.blogspot.com/2016/12/spell-points-by-formula-5e-variant-rule.html. Must decide when spellcasting is first learned; cannot change.
5.) An attacker unseen by his target has advantage only on melee attack rolls, not ranged attack rolls; however, he does qualify for sneak attack damage at range if unseen despite not having advantage.
6.) Anyone with any weapon can attack vital areas at -5 to-hit for +5 to damage. GWM and Sharpshooter feats merely increase the bonus when you are using those weapons.
7.) Casting a non-bonus-action/non-reaction spell triggers an opportunity attack from any enemies in melee range, unless you have the Warcaster feat. (This replaces the third benefit of Warcaster, about reaction spellcasting.) This attack occurs after the spell is cast but before it takes effect (e.g. can still hit someone Dimension Dooring away, can disrupt a concentrations spell and prevent it from taking effect). If the attacker is a Mage Slayer, they can force a concentration save to potentially disrupt even non-concentration spells.
8.) Casting a non-bonus-action/non-reaction spell while moving at more than half speed, riding a horse or on a moving ship forces a concentration save every round even if it's not a concentration spell (Fireball) or it fizzles. Fizzling does not cost spell points but does waste your action to no effect.
9.) There is no Disengage. Opportunity attacks occur when you move at full speed away from an enemy (turning your back), or whenever you are paralyzed/unconscious. You can back away at half speed without turning your back. Creatures like beholders and black puddings have no backs to turn and can move at full speed in any direction without provoking opportunity attacks.
Remark: Dashing while moving backwards replaces and is equivalent to Disengage. You move half speed ('15), but you do it twice because you Dashed, so you move 30' without provoking opportunity attacks--that's why Disengage does not exist, because it's redundant.
10.) Falling damage doubles for every size category over Medium, and halves for every size category under Small. For example, an Ogre falling 100' would take 20d6 HP of damage, not 10d6, because it is Large; and a Fire Giant falling the same distance would take 40d6 damage because it is Huge; but a housecat would take only 5d6 because it is Tiny, and a rat would take 2d6 because it is Tiny II.
11.) Abilities which recharge on "rolling initiative" instead recharge after five minutes. Specifically the following:
I. Relentless [Battlemaster 15th IIRC]: five minutes after you expend your last superiority die, you regain one die.
II. Perfect Self [Monk 20]: whenever you've had less than four ki for five minutes and haven't spent ki during that time, you regain enough ki to have four ki remaining.
III. Superior Inspiration [Bard 20]: five minutes after you expend your last use of Bardic Inspiration, you regain one use of Bardic Inspiration.
12.) While you are incapacitated/stunned/paralyzed/unconscious (but not grappled/restrained), your Dex is 0. Won't affect PCs in heavy armor, but that swashbuckling rogue is in deep trouble if he ever gets paralyzed by a monster or put to sleep, even briefly.
13.) You do not heal to full health automatically on a long rest. Hit Dice can normally only be gained or spent on a long rest, instead of a short rest, and on any given rest you can spend HD or regain half of your HD but not both. However, Bardic Song of Healing now also allows you to spend one HD during a short rest.
14.) You can go below zero HP. Instead of the normal rules on death saves and stabilization, you die whenever you reach negative (max HP). E.g. if you have 40 max HP normally, you die at -40 HP. When you are at zero HP or below, you are either stunned or unconscious. (If you choose to make a DC 15 Con save and succeed you can be stunned, but if you fail the save or choose not to try, you are unconscious from shock.) When you are below zero HP and are not already stable, you must make a death save at the start of every round. If you succeed, you are stable unless/until you take damage again. If you fail, you take 20% of your max HP in damage, rounded UP, not down. You can be stabilized by another character's actions as usual, through the use of a healer's kit or the Wisdom (Medicine) skill or a Spare the Dying cantrip, and any amount of healing also stabilizes you, even 1 HP.
Example: if you have 40 HP normally, and you get hit twice by an Iron Golem for a total of 50 HP of damage, you're now at -10 HP (and likely unconscious, unless you made the DC 15 Con save). Since you're at -10 HP, not zero HP, you can't be restored to full activity by a simple 1 HP Word of Healing as you would under PHB rules--it takes 11 HP of healing to get you conscious again. At the start of every round, you make a death save (as usual, it is DC 10 and no attribute modifiers apply). If you succeed you stabilize at your current HP, otherwise you lose another 8 HP and must save again next round. If you ever reach -40 HP you die.
Remark: In some ways losing 20% of your HP is more generous than the default rules because it only takes one roll to stabilize, and someone who is just barely at negative HP may take five failures before they die. A wound which takes you down to -1 HP is extremely unlikely to kill you. In other ways though, it is less generous because stabilizing doesn't wipe out past failures--that requires actual healing. Furthermore, if you're deep in the negatives, a single failure will kill you, possibly before anyone else can intervene.
15.) Parry: This is a special type of attack which attacks attacks. When you Attack on your turn, you may choose to dedicate one or more of those attacks to Parrying. If an enemy attacks you with a melee weapon before your next turn, you may roll a melee weapon attack and replace your AC with your attack roll against that attack. You can do this a number of times equal to the number of attacks you dedicated to Parrying.
Example: Robilar the Mighty, an 11th level fighter, has been attacked in his bed by two assassins. Unarmed and unarmored, he snatches up a nearby log to use as an improvised club, and dedicates two of his three attacks to parrying. Robilar inflicts some damage on an assassin with his remaining attack, but then the assassins strike back. On the first assassin's attack, Robilar parries, and rolls d20+8 on his melee attack (for Strength 18 and proficiency bonus +4), getting a total of 23, which he uses instead of his normal unarmored AC of 10. The assassin rolls d20+6, gets a 15, and fails to hit AC 23! Then the second assassin strikes, and Robilar rolls d20+8 and gets a 14. The assassin rolls d20+6 and gets 17, so Robilar is hit! The assassin rolls 5d6+4 poison damage and inflicts 27 HP of damage on Robilar--Robilar is in trouble if he doesn't finish them off soon!
16.) To avoid breaking the game, Simulacrum works more like AD&D Simulacrum than PHB Simulacrum. Instead of an almost-perfect copy of the original, Simulacrum produces a dull, listless imitation of the original. If the original creature has any class levels or special abilities, the copy has only 50% of those class levels or abilities, rounded up (the player can select which ones, e.g. if you copy a dual-classed Fighter 5/Wizard 6, you can pick which feats to keep and if you want a Fighter 5/Wizard 1 or a Wizard 6).
In exchange for this nerf, Simulacrum is now not restricted to humanoids, and it may regain spell slots as normal by resting, but it never increases in power (never gains levels).
Complex changes
1.) Open-ended d20 rolls. Since skill checks and saves, unlike attack rolls, don't auto-succeed on a 20 or auto-fail on a 1, but I always want there to be some chance of failure*, on a 20 you re-roll at +10 and take the highest roll. Roll again at +20 if you roll another 20, etc. If you roll a 1, re-roll at -10 and take the lowest. If it's obvious that you've already failed or succeeded you can of course stop rolling already.
*Unless you have Reliable Talent.
2.) Concurrent multiclassing is an option. With concurrent multiclassing, you can advance in two classes or three at the same time, e.g. you could be a 10th level Battlemaster/Necromancer with the abilities of both a 10th level Battlemaster and a 10th level Necromancer. See https://bluishcertainty.blogspot.com/2017/01/5e-old-school-multiclassing-rules_29.html for more details.
3.) XP awards. All characters get a share of XP proportional to their share of the total levels or CR (rounded up to 1) on their side of a combat. For example, in a party of three 9th level PCs and one 5th level PC (total of 32 levels), if they earn 2000 XP from defeating twenty orcs, the 9th level PCs will all earn 9/32 * 2000 = 562.5 XP, while the 5th level PC earns 5/32 * 2000 = 312.5 XP. But if one of the PCs casts Animate Objects and temporarily animates 10 Tiny Objects during the fight, then there are 42 total levels/CR, so the 9thl level PCs earn only 9/42 * 2000 = 428.6 XP, while the 5th level PC earns 238.1 XP.
Purpose: this rule does not exist to punish you, it exists to keep the game interesting, so that you have a good excuse NOT to make the game too easy by flooding every fight with animated dead, purchased mastiffs, and summoned creatures unless you genuinely need them to survive and beat a tough enemy.
4.) Different initiative variant, WE-GO instead of IGO-UGO, and is designed to enhance player engagement and teamwork by reducing the amount of time players spend waiting for their turn to interact with the DM, while also making more intelligent characters and monsters seem more intelligent.
Procedure: DM secretly decides all monster actions while players consult each other and declare everyone's actions for the round together. Then actions are resolved in an order determined by the DM's best judgment of realism and convenience (e.g. arrows are faster than human feet so an arrow attack may happen before a move-and-melee-attack; but the DM might also resolve them both at the same time if the order isn't likely to change any outcomes), with initiative contests when the DM calls for one to decide potential ties.
Once you've declared an action or movement usage for this round you are committed and can't change it except how you initially specified (e.g. you can declare "I'm charging the goblins (moving towards them and Dashing if necessary) and attacking whoever gets within range if I didn't need to Dash"), but you can delay action declaration (or explicitly declare Delay). At any time before the round ends, you can declare an action or movement usage that you haven't used yet (e.g. "I'm standing up" after someone knocks you down, if you have enough movement left, or "I take cover") but then you automatically lose any initiative contests the DM calls for against those who declared actions before you. When the round ends, the DM will pause briefly for additional declarations, and if none are made (e.g. if a Mexican standoff occurs), any unused actions or movements are lost and a new round begins.
During initial action declaration for the round, faster thinking (a tighter OODA loop) is represented by letting highly intelligent creatures gain extra knowledge about other's actions before acting. A character (or monster) who wishes to observe other creatures before declaring an action may take a penalty of N on any initiative contests this round. If so, that character or monster may learn the action declarations of any characters or monsters with intelligence less than or equal to [character's own Int] - 10 + N, before declaring their own action. Example: if Erac the Mage (Int 17) does Observe 4, Erac's player may ask the DM what any monster with Int 11 or less is doing (which could be Delaying) before Erac has to declare his own action. If Erac chooses to Fireball because a group of goblins is preparing to scatter in all directions, and if the DM decides that an initiative contest is needed to see if the goblins scatter before the Fireball detonates, Erac will have -4 on that initiative contest because he paused to study the goblins before acting.
True surprise is rare and occurs only when an unwary target has effectively declared a non-combat action such as "read a book" at the same time a hidden attacker is preparing to attack them. If a target is wary (e.g. an adventurer in a dangerous dungeon) but unaware of a specific threat (the goblin aiming a crossbow at his back), at the start of combat the attacker will declare an action, and the target will be treated as having implicitly Delayed and will get to declare an action after the attacker's action is resolved.
Ask DM for details (or consult brief writeup at https://bluishcertainty.blogspot.com/2017/01/simultaneous-initiative-in-5e.html).
5.) Magic Resistance and Legendary Resistance works differently--requires a reaction and can dispel a spell it's affected by, regardless of whether or not it has a save. Details here: https://bluishcertainty.blogspot.com/2016/03/5e-magic-resistance-variant-rule.html Fundamentally, instead of advantage on saves, it's now like Dispel Magic as a reaction whenever a spell would directly or indirectly affect the monster.
If a creature attempts to use its magic resistance against a given spell and fails, that represents being unable to resist this casting of that spell unless its magic resistance improves--any retries will result in failure. E.g. if you've got a demon bound with Planar Binding, the demon gets only one chance to resist that Planar Binding. (But a crafty demon may not test the Planar Binding right away so be on your guard.)
If magic resistance fails due to temporary circumstances like Hex or Cutting Words, that represents a temporary failure which can be overcome if the creature retries without the hindance. In this rare circumstance, the DM may record the original d20 roll prior to the temporary modifiers, and re-use it on the subsequent attempts. (Or the DM may choose another equivalent method with the same probability curve.) Ditto for temporary improvements: a demon which rolls a 7 (failure) on its MR check against Planar Binding, but then receives Enhance Ability (Charisma) and tests the spell again, would roll one new die, compare it to the previous 7, and take the higher result.
--
I could not love thee dear, so much,
Loved I not honor more.
I could not love thee dear, so much,
Loved I not honor more.
