You control at least two additional nontoken Hydras.
Polukranos on the battlefield as Polukranos, Engine of Ruin.
Steps
Activate Ashnod's Altar by sacrificing another nontoken Hydra that does not have Nim Deathmantle attached, adding ({C} magic symbol) ({C} magic symbol) .
When the Hydra dies, Polukranos and Nim Deathmantle trigger.
Resolve the Polukranos trigger, creating two 3/3 Phyrexian Hydra creature tokens.
Activate Ashnod's Altar by sacrificing a Phyrexian Hydra, 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 nontoken Hydra from your graveyard to the battlefield, and attach Nim Deathmantle to it.