User blog comment:Sithlord Andeddu/Irene's scheme/@comment-88.145.191.141-20160810195135/@comment-7418318-20160810235509

Some magical artifact most likely, though depending on just how depraved Irene is, it could just be something mundane like valuable jewelry or a mansion I suppose.

The magical item would make more sense, since it could be enchanted in a way that only allows the legitimate heir to use it, which will only be Irene once Erza's dead. But if the master enchantress can't undo such an enchantment it must be one hell of a spell that their ancestors put on the thing in question.