Mortuary + Bolas's Citadel + Proper Burial + Blood Pet

Mortuary

Bolas's Citadel

Proper Burial

Blood Pet

How it works

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

This clever engine turns a humble mana-producer into an infinite loop by exploiting life as a resource. With Bolas's Citadel allowing you to cast spells directly from the top of your library for life instead of mana, you can repeatedly sacrifice Blood Pet to generate black mana, only for Mortuary to instantly tuck it back onto the top of your library when it hits the graveyard. Meanwhile, Proper Burial completely neutralizes the life payment of Bolas's Citadel by giving you back life equal to the sacrificed creature's toughness every single iteration, leaving you free to cycle the loop indefinitely for infinite sacrifices, death triggers, and black mana.