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

Toph, the First Metalbender

Ugin's Nexus

Meticulous Excavation

Ashnod's Altar

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 Ashnod's Altar by sacrificing Ugin's Nexus, adding CC. 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. Activate Meticulous Excavation by paying 2, returning Ugin's Nexus from the battlefield to your hand.
6. Repeat each turn.

This ingenious loop turns Ugin's Nexus into an endless engine for infinite turns by cleverly leveraging the permanent-type transformations provided by Toph, the First Metalbender. Because Toph turns all your nontoken artifacts into lands, her end-step trigger can target Ugin's Nexus, turning it into a land creature that can then be fed directly to Ashnod's Altar for mana while triggering its replacement effect to grab an extra turn and exile itself (which Toph immediately bypasses by returning it to the battlefield). To complete the loop and reset the Nexus back to your hand, you simply spend mana on Meticulous Excavation's activation cost, allowing you to recast it and repeat the whole sequence every single turn.