Mortuary + Cauldron Familiar + Bolas's Citadel + Carrion Feeder

Mortuary

Cauldron Familiar

Bolas's Citadel

Carrion Feeder

How it works

1. Cast Cauldron Familiar either from your hand or from the top of your library by paying or 1 life respectively.
2. When Cauldron Familiar enters, it triggers, causing each opponent to lose 1 life and you to gain 1 life.
3. Activate Carrion Feeder by sacrificing Cauldron Familiar.
4. When Cauldron Familiar dies, Mortuary triggers, putting Cauldron Familiar from your graveyard onto the top of your library.
5. Resolve the Carrion Feeder ability.
6. Repeat.

This wicked loop turns the top of your library into an endless revolving door of life drain and growth. With Bolas's Citadel allowing you to cast spells directly from your library by paying life instead of mana, you can deploy Cauldron Familiar for a mere 1 life while its entering-the-battlefield trigger immediately gains it back, draining your opponents in the process. When you sacrifice that same Cauldron Familiar to Carrion Feeder, Mortuary steps in to instantly intercept the cat from reaching the graveyard and place it right back on top of your library, perfectly positioned for the Citadel to play it all over again.