Mortuary + Phyrexian Altar + Bolas's Citadel + Serrated Scorpion

Mortuary

Phyrexian Altar

Bolas's Citadel

Serrated Scorpion

How it works

1. Activate Phyrexian Altar 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. Cast Serrated Scorpion from the top of your library by paying 1 life due to Bolas's Citadel.
6. Repeat.

This loop transforms your library into an endless loop of recycled life and doom by turning your top card into a revolving door. With Bolas's Citadel letting you cast spells directly from the top of your library for life instead of mana, you can repeatedly deploy Serrated Scorpion as long as you have the life to spare. Every time you feed the creature to Phyrexian Altar to generate a mana of any color, Mortuary steps in to instantly catch the card and place it right back on top of your library. Meanwhile, Serrated Scorpion's own death trigger keeps draining your opponents while netting you life to offset the Citadel's cost, fueling the loop indefinitely until the table is bested.