You control at least one additional nontoken creature.
Steps
Activate Ashnod's Altar by sacrificing another nontoken creature, adding ({C} magic symbol) ({C} magic symbol) .
When the creature dies, Sek'Kuar and Nim Deathmantle trigger.
Resolve the Sek'Kuar trigger, creating a 3/1 Graveborn creature token with haste.
Activate Ashnod's Altar by sacrificing the Graveborn, 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 sacrificed creature from your graveyard to the battlefield, and attach Nim Deathmantle to it.