Kaya the Inexorable, Ugin's Nexus, Karn, the Great Creator, and Phyrexian Altar

Prerequisites

• Kaya the Inexorable on the battlefield
• Ugin's Nexus on the battlefield
• Karn, the Great Creator on the battlefield
• Phyrexian Altar on the battlefield
• each extra turn available

Steps

1. 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
2. Activate Kaya's first loyalty ability by putting a loyalty counter on it, putting a ghostform counter on Ugin's Nexus
3. Activate Phyrexian Altar by sacrificing Ugin's Nexus, adding one mana of any color
4. Ugin's Nexus is exiled instead of being put into your graveyard, causing you to take an extra turn after this one
5. Ugin's Nexus triggers, returning itself from exile to your hand, and creating a 1/1 Spirit creature token
6. Move to your next turn
7. Activate Phyrexian Altar by sacrificing the Spirit token, adding one mana of any color
8. Cast Ugin's Nexus by paying
9. Repeat

Results

• Infinite turns