Osgir, the Reconstructor + Karn, the Great Creator + Ugin's Nexus + Aeromoeba + Exalted Sunborn

Osgir, the Reconstructor

Karn, the Great Creator

Ugin's Nexus

Aeromoeba

Exalted Sunborn

How it works

1. Activate Osgir by paying 5, tapping it and exiling Ugin's Nexus from your graveyard, creating four token copies of Ugin's Nexus.
2. Put three of the Ugin's Nexus tokens into exile due to the legend rule and their replacement effects, causing you to take three extra turns after this one.
3. Activate Karn's first loyalty ability by putting a loyalty counter on it, choosing to do nothing.
4. Move to your next extra turn.
5. Repeat from step 3 one additional time.
6. Activate Karn's second loyalty ability by removing two loyalty counters from it, returning Ugin's Nexus from exile to your hand.
7. Activate Aeromoeba by discarding Ugin's Nexus.
8. Move to your last extra turn.
9. Repeat.

This engine revolves around exploiting Ugin's Nexus's replacement effect when it goes to the graveyard, bypassing the legend rule by creating multiple copies with Osgir, the Reconstructor to stockpile a massive chain of extra turns. To keep the loop going indefinitely, Karn, the Great Creator reaches into exile to retrieve the original Nexus, while Aeromoeba neatly discards it back into the graveyard so Osgir can repeat the process. Meanwhile, Exalted Sunborn doubles the token output, supercharging Osgir's replication to ensure you have more than enough extra turns to safely lock down the game.