Athreos, Shroud-Veiled + Ugin's Nexus + Karn, Silver Golem + Master Transmuter + Phyrexian Altar

Athreos, Shroud-Veiled

Ugin's Nexus

Karn, Silver Golem

Master Transmuter

Phyrexian Altar

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 Phyrexian Altar by sacrificing Ugin's Nexus, adding .
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 brilliant loop turns Ugin's Nexus into a recursive engine for infinite turns by using Karn, Silver Golem to temporarily animate it so it can be sacrificed to Phyrexian Altar. While the Altar's sacrifice normally sends the Nexus to the graveyard, its own replacement effect exiles it instead—triggering Athreos, Shroud-Veiled to bring the Nexus right back to the battlefield thanks to the coin counter placed during your end step. Finally, Master Transmuter bounces the Nexus back to your hand so you can recast it for five mana on the extra turn granted by the Nexus itself.