Master Transmuter + Ugin's Nexus + Mirrorworks

Master Transmuter

Ugin's Nexus

Mirrorworks

How it works

1. Cast Ugin's Nexus by paying 5.
2. Ugin's Nexus enters the battlefield, triggering Mirrorworks, causing you to pay 2 to create a copy of Ugin's Nexus.
3. Exile the token copy of Ugin's Nexus due to the legend rule, causing you to take an extra turn after this one.
4. Activate Master Transmuter by paying , tapping it, and returning Ugin's Nexus to your hand, choosing not to put an artifact card from your hand onto the battlefield.
5. Repeat each turn.