Activate Undercity Informer by paying {1} and sacrificing Narcomoeba.
When Narcomoeba dies, Mortuary and Pitiless Plunderer trigger.
Resolve the Mortuary trigger, putting Narcomoeba on top of your library from your graveyard.
Resolve the Pitiless Plunderer trigger, creating a Treasure token.
Resolve the Undercity Informer ability, causing you to put Narcomoeba into your graveyard from the top of your library.
Narcomoeba triggers, returning itself to the battlefield from your graveyard.
Activate the Treasure by tapping and sacrificing it, adding {1}.
Repeat.

Infinite death triggers

Infinite creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Infinite self-mill

Mortuary

Narcomoeba

Undercity Informer

Pitiless Plunderer

Cast Mana Severance for {1}{U}, exiling all lands in your library.
Activate Undercity Informer for {1}, milling your library.
Cast Unburial Rites for {3}{W}, returning Thassa's Oracle to the battlefield.
Thassa's Oracle ETBs, winning the game.

Win the game

Mana Severance

Unburial Rites

Thassa's Oracle

Cast Mana Severance by paying {1}{U}, exiling all lands from your library.
Cast Recross the Paths by paying {2}{G}, revealing your entire library and reordering it with Collected Company on top and Thassa's Oracle and Undercity Informer directly below it.
Draw a card by any method.
Cast Collected Company by paying {3}{G}, putting Thassa's Oracle and Undercity Informer from your library onto the battlefield.
Thassa's Oracle triggers.
In response to the trigger, activate Undercity Informer by paying {1} and sacrificing itself, milling your library.
Resolve the Thassa's Oracle trigger, winning the game.

Recross the Paths

Collected Company

Activate Undercity Informer by paying {1} and sacrificing Narcomoeba.
When Narcomoeba dies, Mortuary and Pawn of Ulamog trigger.
Resolve the Mortuary trigger, putting Narcomoeba on top of your library from your graveyard.
Resolve the Pawn of Ulamog trigger, creating a 0/1 Eldrazi Spawn creature token.
Resolve the Undercity Informer ability, causing you to put Narcomoeba into your graveyard from the top of your library.
Narcomoeba triggers, returning itself to the battlefield from your graveyard.
Activate the Eldrazi Spawn by sacrificing it, adding {C}.
Repeat.

Pawn of Ulamog

Activate Undercity Informer by paying {1} and sacrificing Narcomoeba.
When Narcomoeba dies, Mortuary and Sifter of Skulls trigger.
Resolve the Mortuary trigger, putting Narcomoeba on top of your library from your graveyard.
Resolve the Sifter of Skulls trigger, creating a 1/1 Eldrazi Scion creature token.
Resolve the Undercity Informer ability, causing you to put Narcomoeba into your graveyard from the top of your library.
Narcomoeba triggers, returning itself to the battlefield from your graveyard.
Activate the Eldrazi Scion by sacrificing it, adding {C}.
Repeat.

Sifter of Skulls

Activate Undercity Informer by paying {1} and sacrificing Narcomoeba.
When Narcomoeba dies, Mortuary triggers, putting Narcomoeba on top of your library from your graveyard.
Resolve the Undercity Informer ability, causing you to put Narcomoeba into your graveyard from the top of your library.
Narcomoeba triggers, returning itself to the battlefield from your graveyard.
When Narcomoeba enters the battlefield, Mana Echoes triggers, adding at least {C}.
Repeat.
If you control at least one other Illusion, this combo will net infinite colorless mana.

Combos with "Undercity Informer"