When Penumbra Spider dies, it and Nim Deathmantle triggers.
Resolve the Penumbra Spider trigger, creating a 2/4 Spider creature token.
Activate Ashnod's Altar by sacrificing the Spider, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Penumbra Spider from your graveyard to the battlefield, and attach Nim Deathmantle to it.