Mortuary + Bolas's Citadel + Yahenni, Undying Partisan + Serrated Scorpion

Mortuary

Bolas's Citadel

Yahenni, Undying Partisan

Serrated Scorpion

How it works

1. Activate Yahenni 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 Yahenni 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 deck into a self-sustaining engine by turning life into a resource to fuel your cast triggers. You use Bolas's Citadel to cast Serrated Scorpion directly from your library, paying a mere 1 life instead of its mana cost thanks to the Citadel's alternative payment. When Yahenni, Undying Partisan sacrifices the Scorpion, the creature dies and immediately triggers Mortuary, which whisks the Scorpion right back to the top of your library so Bolas's Citadel can play it again. Meanwhile, the Scorpion's own death trigger uses its life-draining effect to easily offset the life you spent casting it, letting you grind your opponents down to nothing while growing Yahenni into a massive threat.