D&D 5E DM's: How Do You Justify NPC's Having Magic/Abilities That Don't Exist in the PHB?

[TwoSix]

Go nuts. But, for me, why would I bother? No player will ever have that ability. It's largely useless for a PC any way since, unless you are attacking "classed" individuals, the ability has no effect. Which means that actually, it would be entirely useless against NPC's - NPC's in 5e don't have classes. NPC's in most editions don't have classes.

That's a purely meta-game ability that pretty much only functions against PC's. It wouldn't even function against a PC's pet or summoned creature. Well, other than dealing damage I suppose.

But it makes one heck of a cool ability for an NPC. Worked absolutely fantastically when I used them in my last adventure. Total shock on the faces of the players when you tell them that their main caster just lost all their spell casting ability.

Hell, I'd let a PC have that ability if they wanted, assuming they jumped through the necessary diegetic hoops to get it. It's a 1-3 round partial incapacitate, it isn't like it's that dangerous.