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