When Infested Thrinax enters the battlefield, it triggers, causing you to create a number of 1/1 Saproling creature tokens when a nontoken creature you control dies.
When Infested Thrinax dies, it triggers for each time it entered the battlefield this turn, and Nim Deathmantle triggers.
Resolve all Infested Thrinax triggers, creating a number of 1/1 Saproling creature tokens equal to its power for each trigger.
Holding priority, activate Ashnod's Altar by sacrificing a Saproling, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger, causing you to pay ({4} magic symbol) to return Infested Thrinax from your graveyard to the battlefield, and attach Nim Deathmantle to it.