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