Ashiok's Erasure

If there is no exiled card (perhaps because the exiled spell was a copy of a spell), or if the exiled card leaves exile somehow, Ashiok's Erasure won't stop players from casting spells.

If Ashiok's Erasure leaves the battlefield before its enters-the-battlefield ability has resolved, its last ability triggers and resolves with no effect, then its enters-the-battlefield ability exiles the target spell for the rest of the game.

If Ashiok's Erasure has exiled a split card, your opponents can't cast a spell with the same name as either half of that card.