You control at least one other nontoken artifact creature.
Steps
Activate Ashnod's Altar by sacrificing another nontoken artifact creature, adding ({C} magic symbol) ({C} magic symbol) .
When the artifact creature dies, Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return the artifact creature from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When the artifact creature enters the battlefield, The Prydwen triggers, creating a 2/2 Human Knight creature token.
Activate Ashnod's Altar by sacrificing the Human Knight, adding ({C} magic symbol) ({C} magic symbol) .