Diregraf Horde dies, triggering Nim Deathmantle, causing you to pay ({4} magic symbol) to return Diregraf Horde from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Diregraf Horde enters the battlefield, creating two 2/2 Zombie creature tokens.
Activate Ashnod's Altar by sacrificing a Zombie, adding ({C} magic symbol) ({C} magic symbol) .