Eternal Witness and Mitotic Slime in your graveyard.
Pitiless Plunderer and Ashnod's Altar on the battlefield.
You control at least one non-Human creature.
({4} magic symbol) ({GW} magic symbol) ({B} magic symbol) ({B} magic symbol) plus enough mana to pay for commander tax, if applicable available.
Steps
Cast Nethroi either from your hand or from the command zone for its mutate cost by paying ({4} magic symbol) ({GW} magic symbol) ({B} magic symbol) ({B} magic symbol) plus commander tax, if applicable, putting it on top of a non-Human creature you control.