Activate Ashnod's Altar by sacrificing Ancient Stone Idol, adding ({C} magic symbol) ({C} magic symbol) .
Ancient Stone Idol dies, triggering itself and Nim Deathmantle.
Resolve the Ancient Stone Idol trigger, creating a 6/12 Construct artifact creature token.
Activate Ashnod's Altar by sacrificing the Construct, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Ancient Stone Idol from your graveyard to the battlefield, and attach Nim Deathmantle to it.