- All permanents on the battlefield.
- 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 Phyrexian Altar by sacrificing Ugin's Nexus, adding one mana of any color.
- 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.