Karn, Silver Golem + Erratic Portal + Ugin's Nexus + Mirrorworks

Karn, Silver Golem

Erratic Portal

Ugin's Nexus

Mirrorworks

How it works

1. Cast Ugin's Nexus by paying 5.
2. When Ugin's Nexus enters, Mirrorworks triggers, causing you to pay 2 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 1, causing Ugin's Nexus to become a creature until end of turn.
5. Activate Erratic Portal by paying 1 and tapping it, choosing not to pay and return Ugin's Nexus from the battlefield to your hand.
6. Repeat each turn.