I'd like to replace the standard Energy Drain effect on a few CR3 Wights with the following Spell-Eater effect:
Spell-Eater On a successful Melee attack, the target must make a Will save against a DC equal to 10 + 1/2 the spell-eating creature’s racial HD + the spell-eating creature’s Cha modifier. (Same as Energy Drain). If the target fails the save, one of the following happens:
If they cast spells spontaneously: they lose one of their highest remaining spell slots and the spell-eating creature gains 5 temporary hitpoints (lasting 1 hour)
If they prepare spells: they lose a randomly selected spell from their highest spell slots and the spell-eating creature gains 5 temporary hitpoints (lasting 1 hour)
If they are not a caster, or only have 0th level spells remaining: nothing happens
The goal is to make the creature scarier to casters and less concerning to mundane characters, especially in a campaign where the party can't have a 15 minute adventuring day.
Thus my question is: How does this ability compare to Energy Drain and would it change the CR of a wight? (Bonus question: In general how valuable would this ability be if you were to fold it into the Monster Creation guidelines, so other users could add it to their own monsters?)