When Orzhov Racketeers dies, its afterlife ability and Nim Deathmantle trigger.
Resolve the Orzhov Racketeers trigger, creating two 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 Orzhov Racketeers from your graveyard to the battlefield, and attach Nim Deathmantle to it.