Athreos, Shroud-Veiled + Ugin's Nexus + Karn, Silver Golem + Master Transmuter + Krark-Clan Ironworks

Athreos, Shroud-Veiled

Ugin's Nexus

Karn, Silver Golem

Master Transmuter

Krark-Clan Ironworks

How it works

1. Activate Karn by paying 1, 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 Krark-Clan Ironworks 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 intricate loop puts a lock on the game by chaining endless extra turns through the unique replacement effect of Ugin's Nexus. By using Karn, Silver Golem to temporarily animate the Nexus, it becomes a valid target for Athreos, Shroud-Veiled to tag with a coin counter during your end step. When you sacrifice the Nexus to Krark-Clan Ironworks for mana, its replacement effect exiles it to trigger an extra turn while Athreos simultaneously returns it from exile to the battlefield. Finally, Master Transmuter bounces the Nexus back to your hand so you can recast it on the subsequent extra turn, keeping the engine running indefinitely.