Cast Nethroi either from your hand or from the command zone for its mutate cost by paying {4}{G/W}{B}{B} plus commander tax, if applicable, putting it on top of a non-Human creature you control.
When Nethroi enters the battlefield, it triggers.
Holding priority, activate Viscera Seer by sacrificing Nethroi.
When Nethroi dies, Pitiless Plunderer triggers, creating a Treasure artifact token.
Resolve the Viscera Seer ability, causing you to scry 1.
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 Viscera Seer twice by sacrificing Eternal Witness and Mitotic Slime.
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.
Resolve both Viscera Seer abilities, causing you to scry 1 twice.
Activate Viscera Seer twice by sacrificing two Oozes.
When the Oozes die, they each trigger and Pitiless Plunderer triggers twice.
Resolve both Pitiless Plunderer triggers, creating two Treasure artifact tokens.
Resolve both Ooze triggers, creating four 1/1 Ooze creature tokens.
Resolve both Viscera Seer abilities, causing you to scry 1 twice.
Activate Viscera Seer twice by sacrificing two Oozes.
When the Oozes die, Pitiless Plunderer triggers twice, creating two Treasure artifact tokens.
Resolve both Viscera Seer abilities, causing you to scry 1 twice.
Activate the seven Treasures by tapping and sacrificing them, adding {4}{G/W}{B}{B}.
Repeat.

Infinite colored mana

Infinite creature tokens

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite sacrifice triggers

Infinite scry 1

Infinite storm count

Infinite Treasure tokens

Pitiless Plunderer

Eternal Witness

Mitotic Slime

Nethroi, Apex of Death

Viscera Seer

Cast Nethroi either from your hand or from the command zone for its mutate cost by paying {4}{G/W}{B}{B}, putting it under a non-Human creature you control.
Nethroi triggers, returning Whitemane Lion and Mitotic Slime from your graveyard to the battlefield.
When Whitemane Lion enters, it triggers, returning Nethroi from the battlefield to your hand.
Activate Phyrexian Altar twice by sacrificing Whitemane Lion and Mitotic Slime, adding {B}{B}.
When Mitotic Slime dies, it triggers, creating two 2/2 Ooze creature tokens.
Activate Phyrexian Altar twice by sacrificing the Oozes, adding {1}{G/W}.
When the Oozes die, they trigger, creating four 1/1 Ooze creature tokens.
Activate Phyrexian Altar three times by sacrificing three Oozes, adding {3}.
Repeat.

Infinite recursion of some creature cards in your graveyard

Phyrexian Altar

Whitemane Lion

Combos with "Nethroi, Apex of Death"