Cosi, the Shattered Mind

Devoid (This card has no color.)

When you cast this spell and whenever Cosi, the Shattered Mind attacks, each opponent exiles the top
two cards of their library. For as long as those cards
remain exiled, you may play them, and mana of any
type can be spent to cast those spells.

Cosi can't be blocked except by three or more
creatures.