When Nevinyrral dies, its second ability and Nim Deathmantle trigger.
Resolve the Nevinyrral trigger, choosing not to pay.
Resolve the Nim Deathmantle trigger, causing you to pay ({4} magic symbol) to return Nevinyrral to the battlefield from your graveyard, and attach Nim Deathmantle to it.
When Nevinyrral enters the battlefield, its first ability triggers, creating a tapped 2/2 Zombie creature token for each creature that has died this turn.
Activate Ashnod's Altar by sacrificing a Zombie token, adding ({C} magic symbol) ({C} magic symbol) .