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