Variant rule: Magic Resistance
[This rule replaces both Monstrous Manual Magic Resistance and Legendary Resistance. Creatures with one or the other should be assigned a Magic Resistance ability and score.]
Some extremely powerful creatures strongly resist and disrupt the effects of magical energy. These creatures live and breath arcane energy, and by an act of will they can cause magic to recoil from them like water droplets skittering off a hot griddle.
A creature with Magic Resistance can apply that resistance whenever it is affected by a magical effect by expending its reaction. If there is a saving throw or attack roll involved, it can wait until the result is known before deciding to use Magic Resistance. When Magic Resistance is used, the creature makes an ability check against the DC of the magical effect (similar to Counterspell) and if the check succeeds, the magical effect is negated. Unlike Counterspell, the check need not occur at the instant of spellcasting. A Magic Resistant creature could, for example, attempt to walk through a Wall of Force, and at the instant where the Wall of Force prevents its movement, expend its reaction to dispel the Wall of Force. A Magic Resistant Creature could similarly choose at any time to resist the effects of a Maze spell holding it captive or a Planar Binding spell compelling its obedience.
Example: Esmerelda the Enchantress casts Hold Monster VII on a Balor with DC 18. It rolls an 11 on its saving throw and fails. But before the magic takes hold, the Balor resists the magic! A Balor has +12 to Charisma (Magic Resistance), and Esmerelda cast the spell at 7th level, so the Balor has to make a Charisma (Magic Resistance) roll of DC 17 to avoid being paralyzed. It rolls a 21 and shatters the spell! The spell ends, freeing the Balor and any other creatures targeted by the spell.
Note: because a reaction is required, Magic Resistance cannot be used by creatures who are surprised or incapacitated.
Note 2: because Magic Resistance is an ability check, things which affect ability checks including Hex and Cutting Words do affect Magic Resistance rolls. This is by deliberate mechanical analogy to Counterspell.
Note 3: whether things like a monk's Stunning Strike can be resisted with Magic Resistance depends on how your DM interprets whether they are magical or not. Magic Resistance is not limited strictly to spells but does apply only to things that are fundamentally magic. If the DM rules that magic resistance applies to Stunning Strike, he will also tell you what level spell each strike is equivalent to for purposes of Magic Resistance. It might be appropriate to set the level equal to the monk's proficiency bonus.
Variant: some DMs might want the players to do the rolling instead of the monsters. A mathematically-equivalent formulation to the above is: monster spends its reaction to set a DC equal to 12 + Charisma (Magic Resistance). The player then has to roll to beat that score with a bonus equal to the level of the spell. In the case of Esmerelda, she can roll at +7 to beat the Balor's DC 24. Just as before, there are 4 chances in 20 that the Balor is affected by the spell.
If I esteem mankind to be in error, shall I bear them down? No. I will lift them up, and in their own way too, if I cannot persuade them my way is better; and I will not seek to compel any man to believe as I do, only by the force of reasoning, for truth will cut its own way.
I could not love thee, dear, so much,
Loved I not Honor more.