Carth the Lion, Karn, the Great Creator, Ugin's Nexus, Oath of Teferi, and Phyrexian Altar

Prerequisites

• Carth the Lion on the battlefield
• Karn, the Great Creator on the battlefield
• Ugin's Nexus on the battlefield
• Oath of Teferi on the battlefield
• Phyrexian Altar on the battlefield
• each extra turn available

Steps

1. Activate Karn's first loyalty ability by putting two loyalty counters on it, causing Ugin's Nexus to become an artifact creature until end of turn
2. Activate Phyrexian Altar by exiling Ugin's Nexus, adding one mana of any color
3. Ugin's Nexus triggers, causing you to take an extra turn after this one
4. Activate Karn's second loyalty ability by removing a loyalty counter from it, returning Ugin's Nexus from exile to your hand
5. Move to your next turn
6. Cast Ugin's Nexus by paying
7. Repeat

Results

• Infinite turns