Karn, Silver Golem, Erratic Portal, Ugin's Nexus, and Ultron, Artificial Malevolence

Prerequisites

• Karn, Silver Golem in the graveyard
• Erratic Portal on the battlefield
• Ugin's Nexus in hand
• Ultron, Artificial Malevolence on the battlefield
• each turn available

Steps

1. Cast Ugin's Nexus by paying
2. When Ugin's Nexus enters, Ultron triggers, causing you to pay to create a token copy of Ugin's Nexus
3. Put the token Ugin's Nexus into exile due to the legend rule and its replacement effect, causing you to take an extra turn after this one
4. Activate Karn by paying , causing Ugin's Nexus to become a creature until end of turn
5. Activate Erratic Portal by paying and tapping it, choosing not to pay and return Ugin's Nexus from the battlefield to your hand
6. Repeat each turn

Results

• Infinite turns
• Lock