Hallowed Spiritkeeper dies, triggering itself and Nim Deathmantle.
Resolve the Hallowed Spiritkeeper trigger, creating at least four 1/1 Spirit creature tokens.
Activate Phyrexian Altar three times by sacrificing three Spirits, adding ({3} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Hallowed Spiritkeeper from your graveyard to the battlefield, and attach Nim Deathmantle to it.