Activate Ashnod's Altar by sacrificing Imperious Oligarch, adding .
When Imperious Oligarch dies, its afterlife ability and Nim Deathmantle trigger.
Resolve the Imperious Oligarch trigger, creating a 1/1 Spirit creature token.
Activate Ashnod's Altar by sacrificing the Spirit, adding .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay to return Imperious Oligarch from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Repeat.