You control an additional nontoken non-Spirit creature.
Steps
Activate Ashnod's Altar by sacrificing a nontoken non-Spirit creature, adding ({C} magic symbol) ({C} magic symbol) .
When the creature dies, Requiem Angel and Nim Deathmantle triggers.
Resolve the Requiem Angel trigger, creating a 1/1 Spirit creature token.
Activate Ashnod's Altar by sacrificing the Spirit, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return the non-Spirit creature from your graveyard to the battlefield, and attach Nim Deathmantle to it.