Abuelo, Ancestral Echo + Ugin's Nexus + Replication Specialist

Abuelo, Ancestral Echo

Ugin's Nexus

Replication Specialist

How it works

1. Activate Abuelo by paying 1, exiling Ugin's Nexus until the beginning of the next end step.
2. At the beginning of your end step, Abuelo triggers, returning Ugin's Nexus from exile to the battlefield.
3. When Ugin's Nexus enters the battlefield, Replication Specialist triggers, causing you to pay 1 to create a token copy of Ugin's Nexus.
4. Put the token Ugin's Nexus into exile due to the legend rule and its replacement effect, causing you to take an extra turn after this one.
5. Activate Abuelo by paying 1, exiling Ugin's Nexus until the beginning of the next end step.
6. Move to your next turn, not skipping it as Ugin's Nexus is not on the battlefield.
7. Repeat from step 2.

This elegant loop turns the legendary restriction of Ugin's Nexus into an engine for infinite turns. By using Abuelo, Ancestral Echo to repeatedly exile and return the original Ugin's Nexus, you trigger Replication Specialist to churn out token copies. The legend rule immediately sends the token to the graveyard, bypassing the Nexus's restriction against extra turns and safely triggering its replacement effect to grant you an extra turn.