Mortuary + Bolas's Citadel + Carrion Feeder + Serrated Scorpion

Mortuary

Bolas's Citadel

Carrion Feeder

Serrated Scorpion

How it works

1. Activate Carrion Feeder 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 Carrion Feeder ability.
6. Cast Serrated Scorpion from the top of your library by paying 1 life due to Bolas's Citadel.
7. Repeat.

This loop transforms the top of your library into an endless engine of life and death, turning Bolas's Citadel into a twisted casting-and-draining machine. By using Carrion Feeder to devour Serrated Scorpion over and over, you trigger Serrated Scorpion's death ability for steady damage and lifegain while Mortuary quietly catches the insect in your graveyard and places it right back onto the top of your library. Bolas's Citadel then allows you to cast Serrated Scorpion straight from the library, letting you pay a tiny bit of life rather than its actual mana cost—a cost easily offset by the life you're draining back.