Elenda dies, triggering its second ability and Nim Deathmantle.
Resolve the Elenda trigger, creating at least one 1/1 Vampire creature token.
Resolve the Nim Deathmantle trigger, causing you to pay ({4} magic symbol) to return Elenda from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Activate Ashnod's Altar by sacrificing a Vampire token, adding ({C} magic symbol) ({C} magic symbol) .
The Vampire dies, triggering Elenda's first ability, putting a +1/+1 counter on Elenda.