Stormonu
NeoGrognard
One option might be "if the magical effect does not allow a save or make an attack roll to hit, the creature gains a save, as an <Ability> save.
Another would be to make it (semi-)separate from a save, at a static DC that is checked first before the save/attack roll is made. If trying to mimic old magic resistance, the caster's level would affect the DC the creature needs to achieve.
Last option I could think of would be to just make it another defense, like AC or a Saving Throw. The creature can require the spell to either use the normal method (vs. AC or Saving Throw) or use the Magic Defense (whichever is better) and the caster uses level or proficiency bonus to "attack" that value.
ex. Creature has a Wis save of +3, AC 15 and a MR 17. If a wizard casts firebolt that would target the AC of 15, the creature can instead force it to target the MR of 17. Instead of making a Wisdom save against charm monster, the caster would have to make an attack against the MR of 17. Against a magic missile, the caster doesn't hit automatically, but instead makes an attack against MR 17.
Another would be to make it (semi-)separate from a save, at a static DC that is checked first before the save/attack roll is made. If trying to mimic old magic resistance, the caster's level would affect the DC the creature needs to achieve.
Last option I could think of would be to just make it another defense, like AC or a Saving Throw. The creature can require the spell to either use the normal method (vs. AC or Saving Throw) or use the Magic Defense (whichever is better) and the caster uses level or proficiency bonus to "attack" that value.
ex. Creature has a Wis save of +3, AC 15 and a MR 17. If a wizard casts firebolt that would target the AC of 15, the creature can instead force it to target the MR of 17. Instead of making a Wisdom save against charm monster, the caster would have to make an attack against the MR of 17. Against a magic missile, the caster doesn't hit automatically, but instead makes an attack against MR 17.