Mortuary + Bolas's Citadel + Serrated Scorpion + Phantom Train

Mortuary

Bolas's Citadel

Serrated Scorpion

Phantom Train

How it works

1. Activate Phantom Train by sacrificing Serrated Scorpion.
2. When Serrated Scorpion dies, it and Mortuary triggers.
3. Resolve the Mortuary trigger, putting Serrated Scorpion from your graveyard onto the top of your library.
4. Resolve the Serrated Scorpion trigger, causing you to gain 2 life and dealing 2 damage to each opponent.
5. Resolve the Phantom Train ability.
6. Cast Serrated Scorpion from the top of your library by paying 1 life due to Bolas's Citadel.
7. Repeat.

This loop turns your library into an endless loop of recycled life drain by leveraging Bolas's Citadel to cast creatures directly off the top without spending mana. Whenever you sacrifice Serrated Scorpion to fuel Phantom Train's animation ability, the Scorpion's death triggers the life swing while Mortuary immediately catches it from the graveyard and tucks it right back onto the top of your library. Bolas's Citadel then lets you recast the Scorpion for a mere 1 life, a cost easily offset by the life you gain on resolution, allowing you to repeat the cycle indefinitely until your opponents are drained away.