Thul, the Sprouting Sequence

Emerge {4}{b}{g} (You may cast this spell by sacrificing a creature and paying the emerge cost reduced by that creature’s mana value.)

You may look at the top card of your library. You
may play lands and cast creature spells for their emerge cost from the top of your library.

Creature cards on top of your library gain emerge. Their emerge cost is equal to their mana cost.