Athreos, Shroud-Veiled + Ugin's Nexus + Karn, the Great Creator + 2 more

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.

At the beginning of your end step, Athreos' second ability and Nahiri's Resolve trigger.

Resolve the Athreos trigger, putting a coin counter on Ugin's Nexus.

Holding priority, activate Krark-Clan Ironworks by sacrificing Ugin's Nexus, adding .

Ugin's Nexus is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.

Athreos' third ability triggers, causing you to return Ugin's Nexus from exile to the battlefield.

Resolve the Nahiri's Resolve trigger from step 2, exiling Ugin's Nexus until your next upkeep.

Move to your extra turn.

At the beginning of your upkeep, Nahiri's Resolve triggers, returning Ugin's Nexus from exile to the battlefield.

Repeat.