Mortuary + Bolas's Citadel + Viscera Seer + Vengeful Bloodwitch

Mortuary

Bolas's Citadel

Viscera Seer

Vengeful Bloodwitch

How it works

1. Activate Viscera Seer by sacrificing it.
2. When Viscera Seer dies, Mortuary and Vengeful Bloodwitch trigger.
3. Resolve the Mortuary trigger, putting Viscera Seer from your graveyard onto the top of your library.
4. Resolve the Vengeful Bloodwitch trigger, causing you to gain 1 life and target opponent to lose 1 life.
5. Resolve the Viscera Seer ability.
6. Cast Viscera Seer from the top of your library by paying 1 life.
7. Repeat.

This clever loop relies on Bolas's Citadel to let you cast your creatures straight from the top of your library by trading life instead of mana. When you use Viscera Seer to sacrifice itself, Mortuary immediately intervenes to place that creature right back onto the top of your library so you can keep recasting it. Meanwhile, Vengeful Bloodwitch watches the carnage unfold, turning every single sacrifice into a steady stream of life loss for your opponents and life gain for you.