Activate Ashnod's Altar by sacrificing Knight of the Last Breath, adding ({C} magic symbol) ({C} magic symbol) .
When Knight of the Last Breath dies, its afterlife ability and Nim Deathmantle trigger.
Resolve the Knight of the Last Breath trigger, creating three 1/1 Spirit creature tokens.
Activate Ashnod's Altar by sacrificing a 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 Knight of the Last Breath from your graveyard to the battlefield, and attach Nim Deathmantle to it.