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

Ugin's Nexus

Aetheric Amplifier

Karn, the Great Creator

Grinding Station

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 Grinding Station 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 loop relies on recycling Ugin's Nexus through its own replacement effect and Karn, the Great Creator's ability to fetch it right back out of exile. By sacrificing Ugin's Nexus to Grinding Station, you trigger the Nexus's exile clause to bank an extra turn while untapping the Station in the process. Aetheric Amplifier then doubles Karn's loyalty counters so Karn can repeatedly use his minus ability to return that same Ugin's Nexus from exile straight back to your hand, ready to cast again.