Cast Nethroi for its mutate cost by paying ({4} magic symbol) ({GW} magic symbol) ({B} magic symbol) ({B} magic symbol) , putting it on top of a non-Human creature you control.
When Nethroi enters the battlefield, it triggers.
Holding priority, activate Thermopod's second ability by sacrificing Nethroi, adding ({R} magic symbol) .
When Nethroi dies, Pitiless Plunderer triggers, creating a Treasure artifact token.
Resolve the Nethroi trigger from step 2, returning Eternal Witness and Mitotic Slime from your graveyard to the battlefield.
When Eternal Witness enters the battlefield, it triggers, returning Nethroi from your graveyard to your hand.
Activate Thermopod's second ability twice by sacrificing Eternal Witness and Mitotic Slime, adding ({R} magic symbol) ({R} magic symbol) .
When Eternal Witness and Mitotic Slime die, Pitiless Plunderer triggers twice and Mitotic Slime triggers once.
Resolve both Pitiless Plunderer triggers, creating two Treasure artifact tokens.
Resolve the Mitotic Slime trigger, creating two 2/2 Ooze creature tokens.
Activate Thermopod's second ability by sacrificing an Ooze, adding ({R} magic symbol) .
When the Ooze dies, it and Pitiless Plunderer trigger.
Resolve the Pitiless Plunderer trigger, creating a Treasure artifact token.
Resolve the Ooze trigger, creating two 1/1 Ooze creature tokens.
Activate three Treasures by tapping and sacrificing them, adding ({GW} magic symbol) ({B} magic symbol) ({B} magic symbol) .