Athreos, Shroud-Veiled + Ugin's Nexus + Karn, the Great Creator + Master Transmuter + Ashnod's Altar

Athreos, Shroud-Veiled

Ugin's Nexus

Karn, the Great Creator

Master Transmuter

Ashnod's Altar

How it works

1. Activate Karn's first loyalty ability by putting a loyalty counter on it, causing Ugin's Nexus to become an artifact creature until end of turn.
2. At the beginning of your end step, Athreos' second ability triggers, putting a coin counter on Ugin's Nexus.
3. Activate Ashnod's Altar by sacrificing Ugin's Nexus, adding CC.
4. Ugin's Nexus is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
5. Athreos' third ability triggers, causing you to return Ugin's Nexus from exile to the battlefield.
6. Activate Master Transmuter by paying , tapping it and returning Ugin's Nexus to your hand, choosing to not put an artifact onto the battlefield.
7. Move to your extra turn.
8. Cast Ugin's Nexus by paying 5.
9. Repeat.

This infinite turn loop hinges on continuously recycling Ugin's Nexus to generate endless extra turns while locking out your opponents. Karn, the Great Creator plays a crucial role by temporarily turning Ugin's Nexus into an artifact creature, allowing Athreos, Shroud-Veiled to place a coin counter on it during your end step and return it whenever it changes zones. You then sacrifice the Nexus to Ashnod's Altar for mana, triggering its replacement effect to exile itself and grant you an extra turn. Finally, Master Transmuter bounces the Nexus back to your hand (or you simply recast it), allowing you to replay the artifact and repeat the entire sequence on every extra turn.