Mortuary + Bolas's Citadel + Blood Pet + Overgrowth Elemental

Mortuary

Bolas's Citadel

Blood Pet

Overgrowth Elemental

How it works

1. Activate Blood Pet by sacrificing it.
2. When Blood Pet dies, Mortuary and Overgrowth Elemental trigger.
3. Resolve the Mortuary trigger, putting Blood Pet from your graveyard onto the top of your library.
4. Resolve the Overgrowth Elemental 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 tiny sacrifice into an infinite life and mana loop by exploiting the top of your library as a hidden hand. Bolas's Citadel does the heavy lifting, allowing you to cast Blood Pet directly from the top of your library by paying life instead of mana. When you sacrifice Blood Pet for black mana, Mortuary instantly intercepts it on the way to the graveyard, placing it right back on top of your library to repeat the cycle. Meanwhile, Overgrowth Elemental cushions the life-payment cost by triggering on the creature's death to keep your life total safely out of the red.