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.
Activate Thermopod's second ability by sacrificing Infested Thrinax, adding ({R} magic symbol) .
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 Thermopod's second ability three times by sacrificing three Saprolings, adding ({R} magic symbol) ({R} magic symbol) ({R} 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.