Mortuary + Cauldron Familiar + Bolas's Citadel + Phantom Train

Mortuary

Cauldron Familiar

Bolas's Citadel

Phantom Train

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 Phantom Train 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 Phantom Train ability.
6. Repeat.

This engine transforms the top of your library into an endless loop of life-draining recurrence. Thanks to Bolas's Citadel, you can cast Cauldron Familiar straight off the top of your library by paying 1 life instead of its mana cost, immediately triggering its life drain and gain upon entry. You then feed that same cat to Phantom Train's activation, turning the Vehicle into an active creature while sending the Familiar to the graveyard. That death instantly triggers Mortuary, placing Cauldron Familiar right back on top of your library so Bolas's Citadel can play it all over again.