You control at least one nontoken artifact creature.
({2} magic symbol) available.
Steps
Activate Ashnod's Altar by sacrificing a nontoken artifact creature, adding ({C} magic symbol) ({C} magic symbol) .
The creature dies, triggering Nim Deathmantle, causing you to pay ({4} magic symbol) to return the artifact creature from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Imotekh's first ability triggers, creating two 2/2 Necron Warrior artifact creature tokens.
Activate Ashnod's Altar by sacrificing a Necron Warrior, adding ({C} magic symbol) ({C} magic symbol) .