Target opponent reveals their hand. You choose a nonland card in that player's hand or graveyard, and exile it. For as long as that card remains exiled, you may cast it, and you may spend mana as though it were any type to cast it.
Not sure how good it is. I suppose the graveyard-clause gives you somewhat of a safety-net so you can find a spot to fire it off where it won't be entirely wasted.
Probably good in the same games/metas/cubes/situations where Mind Rot is good?