Ugin's Nexus + Aetheric Amplifier + Karn, the Great Creator + Ravenous Intruder

Ugin's Nexus

Aetheric Amplifier

Karn, the Great Creator

Ravenous Intruder

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 Ravenous Intruder 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 clever engine achieves infinite turns by repeatedly looping Ugin's Nexus through exile, the battlefield, and your hand. While Ravenous Intruder provides the necessary sacrifice outlet to send Ugin's Nexus to exile—triggering its replacement effect to grant you an extra turn—Karn, the Great Creator acts as the mechanical core by using its −2 ability to pull Ugin's Nexus back from exile into your hand. To keep Karn from running out of fuel, Aetheric Amplifier steps in to double Karn's loyalty counters so you can constantly repeat the process.