When Altar of the Wretched enters the battlefield, you may sacrifice a nontoken creature. If you do, draw X cards, then mill X cards, where X is that creature's power.

Craft with one or more creatures {2}{B}{B}

{2}{B}: Return Altar of the Wretched from your graveyard to your hand.

Wretched Bonemass's power and toughness are each equal to the total power of the exiled cards used to craft it.

Wretched Bonemass has flying as long as an exiled card used to craft it has flying. The same is true for first strike, double strike, deathtouch, haste, hexproof, indestructible, lifelink, menace, protection, reach, trample, and vigilance.