Ugin's Nexus + Aetheric Amplifier + Karn, the Great Creator + Umbral Collar Zealot

Ugin's Nexus

Aetheric Amplifier

Karn, the Great Creator

Umbral Collar Zealot

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 Umbral Collar Zealot 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 setup achieves infinite turns by recycling Ugin's Nexus through exile and back to your hand, using its own replacement effect to trigger the extra turn when sacrificed to Umbral Collar Zealot. To make this loop repeatable without running out of loyalty, Karn, the Great Creator provides the essential recursion by pulling the Nexus back from exile with his minus ability. Since Karn needs to build up counters to keep doing this, Aetheric Amplifier steps in to effortlessly double those loyalty counters for just four mana, while Karn's plus ability can even be used defensively to tick him up when needed.