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

Ugin's Nexus

Aetheric Amplifier

Karn, the Great Creator

Megatog

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 Megatog 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 intricate loop transforms a massive artifact sacrifice into a perpetual sequence of extra turns. The engine relies on Megatog to feed Ugin's Nexus to the graveyard, triggering the Nexus's replacement effect to exile itself and grant you an extra turn. From there, Karn, the Great Creator plays the crucial role of reaching into exile to retrieve Ugin's Nexus, a recovery fueled by the mana from Aetheric Amplifier. To ensure Karn can repeatedly use his minus ability without dying, Aetheric Amplifier's second mode doubles the loyalty counters on Karn, allowing the Planeswalker to sustain the loop indefinitely.