Workshop Warchief dies, triggering itself and Nim Deathmantle.
Resolve the Workshop Warchief trigger, creating a 4/4 Beast creature token.
Activate Ashnod's Altar by sacrificing the Beast, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Workshop Warchief from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Workshop Warchief enters the battlefield, causing you to gain three life.