Ugin's Nexus, Aetheric Amplifier, Karn, the Great Creator, and Phantom Train

Prerequisites

• Ugin's Nexus on the battlefield
• Aetheric Amplifier on the battlefield
• Karn, the Great Creator on the battlefield
• Phantom Train on the battlefield
• each extra turn available

Steps

1. If Karn has exactly one loyalty counter on it, activate its first loyalty ability by putting a loyalty counter on it
2. Activate Phantom Train by sacrificing Ugin's Nexus
3. Ugin's Nexus will be exiled instead of being put into your graveyard due to its replacement effect, causing you to take an extra turn after this one
4. Move to your next turn
5. Activate Aetheric Amplifier by paying and tapping it, doubling the number of counters on Karn
6. Activate Karn's second loyalty ability by removing two loyalty counters from it, returning Ugin's Nexus from exile to your hand
7. Cast Ugin's Nexus by paying
8. Repeat from step 2

Results

• Infinite turns