Golgari Thug + Umbral Collar Zealot + Bolas's Citadel + Vein Ripper

Golgari Thug

Umbral Collar Zealot

Bolas's Citadel

Vein Ripper

How it works

1. Activate Umbral Collar Zealot by sacrificing Golgari Thug.
2. When Golgari Thug dies, it and Vein Ripper trigger.
3. Resolve the Golgari Thug trigger, putting it from your graveyard onto the top of your library.
4. Resolve the Vein Ripper trigger, causing you to gain 2 life and target opponent to lose 2 life.
5. Resolve the Umbral Collar Zealot ability.
6. Cast Golgari Thug from the top of your library by paying 2 life.
7. Repeat.

This loop turns the top of your library into a self-sustaining engine by letting you repeatedly cast a single creature without ever spending mana. With Bolas's Citadel allowing you to play Golgari Thug directly from the top of your library using life instead of mana, Umbral Collar Zealot provides the necessary sacrifice outlet to send it right back to the graveyard. Each time the Thug dies, its own triggered ability puts it neatly back onto the top of your library while simultaneously fueling Vein Ripper's life-drain triggers to offset the life you spent casting it.