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

Ugin's Nexus

Aetheric Amplifier

Karn, the Great Creator

Phantom Train

How it works

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. 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.
3. Move to your next turn.
4. Activate Aetheric Amplifier by paying 4 and tapping it, doubling the number of counters on Karn.
5. Activate Karn's second loyalty ability by removing two loyalty counters from it, returning Ugin's Nexus from exile to your hand.
6. Cast Ugin's Nexus by paying 5.
7. Repeat from step 2.

This brilliant engine uses Karn, the Great Creator to constantly recycle Ugin's Nexus from exile back into your hand, bypassing its replacement effect that would normally prevent extra turns. By leveraging Phantom Train to sacrifice the Nexus, you trigger its exile clause and immediately claim an extra turn while fueling the Train's transformation. Once you have looped back around, Aetheric Amplifier steps in to effortlessly double Karn's loyalty counters so he has enough fuel to fetch the Nexus right back out of exile over and over again.