Mortuary + Vein Ripper + Bolas's Citadel + Blood Pet

Mortuary

Vein Ripper

Bolas's Citadel

Blood Pet

How it works

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

This clever loop centers around Bolas's Citadel, which turns the top of your library into your hand and replaces mana costs with life payments. By sacrificing Blood Pet for black mana, you trigger Mortuary to immediately recycle the creature back to the top of your library for the Citadel to replay. Meanwhile, Vein Ripper oversees the whole operation, draining your opponent for 2 life each time Blood Pet dies to comfortably offset the life you spent casting it.