Altar of the Wretched // Wretched Bonemass

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.

When Altar of the Wretched enters, 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.

The Lost Caverns of Ixalan Commander

Craft abilities are activated abilities with costs that have a mana component as well as an additional "materials" component.

Craft abilities are written "Craft with [materials] [mana]", which means "[Mana], Exile this permanent, Exile [materials] from among permanents you control and/or cards in your graveyard: Return this card to the battlefield transformed under its owner's control. Activate only as a sorcery."

If a card that isn't a transforming double-faced card becomes a copy of a card with craft, it'll stay in exile if you activate the craft ability. It won't return to the battlefield.

If any of the exiled cards has a characteristic-defining ability that defines its power and/or toughness, that ability will apply. For example, if you used Souls of the Lost to craft Wretched Bonemass, its power and toughness while in exile are determined by the number of permanent cards in your graveyard, and Wretched Bonemass's power and toughness will change as the number of permanent cards in your graveyard changes. If the characteristic-defining ability can't be applied, then use 0.

If the materials required include multiple objects, you may exile some of them from among permanents you control and the rest from among cards in your graveyard. You don't have to choose all permanents or all cards from your graveyard.

The back faces of some cards with craft refer to cards "used to craft" it. This refers to the cards exiled as part of the cost of the craft ability of the front face. Those cards are considered to be "used to craft" that permanent as long as they remain exiled and the permanent remains on the battlefield, even if the permanent's controller changes or some of its characteristics change (because of a copy effect, for example.)

You may exile tokens you control as part of the materials required. However, because they aren't cards and won't stay in exile, any abilities that refer to what you "used to craft" the back faces won't refer to anything.