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

Ugin's Nexus

Aetheric Amplifier

Karn, the Great Creator

Arcbound Ravager

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 Arcbound Ravager 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 achieves infinite turns by continuously cycling Ugin's Nexus in and out of exile to bank extra turns. The engine relies on Arcbound Ravager to sacrifice the Nexus, triggering its replacement effect to exile itself and grant you an extra turn while sending it to the exile zone rather than the graveyard. Once safely in exile, Karn, the Great Creator uses its minus ability to retrieve Ugin's Nexus right back to your hand so it can be recast. To fuel this recursion indefinitely without running out of loyalty, Aetheric Amplifier is tapped and activated for four mana to double Karn's counters, ensuring the planeswalker always has enough fuel to keep pulling the Nexus out of exile.