Nethroi, Apex of Death | Phyrexian Altar | Mitotic Slime

(and one other card)

({4} magic symbol) ({GW} magic symbol) ({B} magic symbol) ({B} magic symbol) plus enough mana to pay for commander tax, if applicable available.

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) , putting it under a non-Human creature you control.
Nethroi triggers, returning
Cavern Harpy and
Mitotic Slime from your graveyard to the battlefield.
When
Cavern Harpy enters, it triggers, returning
Nethroi from the battlefield to your hand.
Activate
Phyrexian Altar twice by sacrificing
Cavern Harpy and
Mitotic Slime, adding ({B} magic symbol) ({B} magic symbol) .
When
Mitotic Slime dies, it triggers, creating two 2/2 Ooze creature tokens.
Activate
Phyrexian Altar twice by sacrificing the Oozes, adding ({1} magic symbol) ({GW} magic symbol) .
When the Oozes die, they trigger, creating four 1/1 Ooze creature tokens.
Activate
Phyrexian Altar three times by sacrificing three Oozes, adding ({3} magic symbol) .
Repeat.

During step 2, you can recur other creatures with suitable powers. This will allow you to forego sacrificing that many Oozes in step 8, netting infinite mana and creature tokens. If you have infinite mana and Oozes, you can then forego reanimating
Mitotic Slime and reanimate anything else with a suitable power instead.