Mortuary + Zulaport Cutthroat + Bolas's Citadel + Blood Pet

Mortuary

Zulaport Cutthroat

Bolas's Citadel

Blood Pet

How it works

1. Activate Blood Pet by sacrificing it.
2. When Blood Pet dies, Mortuary and Zulaport Cutthroat trigger.
3. Resolve the Mortuary trigger, putting Blood Pet from your graveyard onto the top of your library.
4. Resolve the Zulaport Cutthroat trigger, causing you to gain 1 life and each opponent to lose 1 life.
5. Cast Blood Pet from the top of your library by paying 1 life.
6. Repeat.

This clever loop turns the top of your library into an endless armory, fueled by Bolas's Citadel allowing you to bypass mana costs by trading life instead. When you sacrifice Blood Pet for black mana, Mortuary catches it on the way to the graveyard and immediately places it back onto the top of your library to be played again. Meanwhile, Zulaport Cutthroat turns every iteration of this cycle into a steady stream of life drain and life gain, easily offsetting the life payment required by the Citadel while closing out the game.