Mortuary + Bolas's Citadel + Phyrexian Dreadnought + Sifa Grent

Mortuary

Bolas's Citadel

Phyrexian Dreadnought

Sifa Grent

How it works

1. Cast Phyrexian Dreadnought from the top of your library by paying 1 life.
2. When Phyrexian Dreadnought enters, it triggers, causing you to sacrifice it.
3. When Phyrexian Dreadnought dies, Mortuary and Sifa Grent trigger.
4. Resolve the Mortuary trigger, putting Phyrexian Dreadnought from your graveyard onto the top of your library.
5. Resolve the Sifa Grent trigger, causing you to gain 1 life and target opponent to lose 1 life.
6. Repeat.

This clever loop sets up an endless engine fueled entirely by the top of your library. With Bolas's Citadel granting you the ability to cast Phyrexian Dreadnought directly from your library for just 1 life, the Dreadnought immediately triggers its own sacrifice clause upon entering the battlefield. As it hits the graveyard, Mortuary instantly catches it and puts it right back on top of your library to repeat the process, while Sifa Grent turns every single one of those looping deaths into a drain on your opponents' life totals and a replenishment of your own.