Mortuary + Narcomoeba + Sanguine Spy + Pitiless Plunderer

Mortuary

Narcomoeba

Sanguine Spy

Pitiless Plunderer

How it works

1. Activate Sanguine Spy by paying 1 and sacrificing Narcomoeba.
2. When Narcomoeba dies, Mortuary and Pitiless Plunderer trigger.
3. Resolve the Mortuary trigger, putting Narcomoeba on top of your library from your graveyard.
4. Resolve the Pitiless Plunderer trigger, creating a Treasure token.
5. Resolve the Sanguine Spy ability, causing you to put Narcomoeba into your graveyard from the top of your library.
6. Narcomoeba triggers, returning itself to the battlefield from your graveyard.
7. Activate the Treasure by tapping and sacrificing it, adding 1.
8. Repeat.

This clever loop relies on Sanguine Spy to continually sacrifice Narcomoeba, using the surveil ability to cycle the creature right back off the top of your library. As Narcomoeba hits the graveyard, Mortuary neatly places it back onto your library while Pitiless Plunderer converts that death into a Treasure token. By cracking that Treasure for mana to pay for Sanguine Spy's activation cost, you sustain the entire engine indefinitely.