Abhorrent Overlord dies, triggering Nim Deathmantle, causing you to pay ({4} magic symbol) to return Abhorrent Overlord from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Abhorrent Overlord enters the battlefield, creating a number of 1/1 Harpy creature tokens equal to your devotion to black.
Activate Ashnod's Altar by sacrificing a Harpy, adding ({C} magic symbol) ({C} magic symbol) .