When Altar of the Wretched enters the battlefield, you may sacrifice a nontoken creature. If you do, draw X cards, then mill (To mill a card, a player puts the top card of their library into their graveyard.) X cards, where X is that creature's power. Craft with one or more creatures : 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 (This creature can't be blocked except by creatures with flying or reach.) as long as an exiled card used to craft it has flying (This creature can't be blocked except by creatures with flying or reach.). The same is true for first strike (This creature deals combat damage before creatures without first strike.), double strike (This creature deals both first-strike and regular combat damage.), deathtouch (Any amount of damage this deals to a creature is enough to destroy it.), haste (This creature can attack, and as soon as it comes under your control.), hexproof (This permanent can't be the target of spells or abilities your opponents control.), indestructible (Effects that say "destroy" don't destroy that permanent. An indestructible creature can't be destroyed by damage.), lifelink (Damage dealt by the creature also causes its controller to gain that much life.), menace (This creature can't be blocked except by two or more creatures.), protection (A permanent or player with protection can't be targeted, enchanted, equipped, fortified, damaged or blocked by sources of the given property.), reach (This creature can block creatures with flying.), trample (This creature can deal excess combat damage to the player or planeswalker it's attacking.), and vigilance (Attacking doesn't cause this creature to tap.).