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

Toph, the First Metalbender

Ugin's Nexus

Meticulous Excavation

Megatog

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 Megatog 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 Megatog ability.
6. Activate Meticulous Excavation by paying 2, returning Ugin's Nexus from the battlefield to your hand.
7. Repeat each turn.

This clever loop centers around Ugin's Nexus, which replaces its own trip to the graveyard with exile to grant you an extra turn. Toph, the First Metalbender makes your nontoken artifacts into lands, meaning Ugin's Nexus is treated as a land and can be conveniently bounced back to your hand with Meticulous Excavation's ability if it goes to the battlefield, or returned via Toph's own earthbend trigger if it hits exile. To get the Nexus into the graveyard in the first place, you feed it to Megatog, sacrificing the artifact to pump up its stats while triggering the replacement effect that secures your endless extra turns.