Toph, the First Metalbender + Ugin's Nexus + Meticulous Excavation + Sarcatog

Toph, the First Metalbender

Ugin's Nexus

Meticulous Excavation

Sarcatog

How it works

1. Cast Ugin's Nexus by paying 5.
2. At the beginning of your end step, Toph triggers, causing Ugin's Nexus to become an artifact land creature.
3. Activate Sarcatog by sacrificing Ugin's Nexus. Ugin's Nexus is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
4. Toph triggers, returning Ugin's Nexus from exile to the battlefield.
5. Resolve the Sarcatog ability.
6. Activate Meticulous Excavation by paying 2, returning Ugin's Nexus from the battlefield to your hand.
7. Repeat each turn.

This ingenious loop locks down the game by turning your artifacts into lands, transforming Ugin's Nexus into a land creature via Toph, the First Metalbender at the beginning of your end step. Once it's a land, you can sacrifice Ugin's Nexus to Sarcatog; its replacement effect exiles it instead of putting it into the graveyard, which triggers an extra turn while Toph simultaneously ensures it returns safely from exile. To keep the engine running, you simply use Meticulous Excavation to bounce Ugin's Nexus back to your hand so you can replay it and repeat the whole process indefinitely.