Crawling Chorus dies, triggering itself and Nim Deathmantle.
Resolve the Crawling Chorus trigger, creating a 1/1 Phyrexian Mite creature token.
Activate Ashnod's Altar by sacrificing the Phyrexian Mite, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Crawling Chorus from your graveyard to the battlefield, and attach Nim Deathmantle to it.