Activate Phyrexian Altar by sacrificing Hallowed Spiritkeeper, adding 1.
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.
Resolve the Nim Deathmantle trigger from step 2, causing you to pay 4 to return Hallowed Spiritkeeper from your graveyard to the battlefield, and attach Nim Deathmantle to it.