(Howltooth of the Burnwillows isn't a spell.)

Howltooth of the Burnwillows enters the battlefield tapped unless you sacrifice a Black permanent.

Whenever a card is put into a graveyard from anywhere, exile that card.

: Add  or to your mana pool.

: Add  to your mana pool. Put Howltooth of the Burnwillows on top of your library.