Activate Ashnod's Altar by sacrificing Knight of Sorrows, adding ({C} magic symbol) ({C} magic symbol) .
When Knight of Sorrows dies, its afterlife ability and Nim Deathmantle trigger.
Resolve the Knight of Sorrows 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 Knight of Sorrows from your graveyard to the battlefield, and attach Nim Deathmantle to it.