The Sixth Doctor + Ugin's Nexus + Meticulous Excavation + Krark-Clan Ironworks

The Sixth Doctor

Ugin's Nexus

Meticulous Excavation

Krark-Clan Ironworks

How it works

1. Cast Ugin's Nexus by paying 5.
2. The Sixth Doctor triggers, copying the Ugin's Nexus spell, with the copy being nonlegendary.
3. Resolve both Ugin's Nexus spells.
4. Activate Krark-Clan Ironworks by sacrificing the Ugin's Nexus token, adding CC.
5. The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
6. Activate Meticulous Excavation by paying 2, returning the nontoken Ugin's Nexus to your hand.
7. Repeat each turn.

This ingenious setup loops extra turns by exploiting the legend rule and replacement effects around Ugin's Nexus. When you cast Ugin's Nexus, The Sixth Doctor steps in to copy it, conveniently bypassing the legendary restriction so both versions can comfortably coexist on your battlefield. You then sacrifice the nonlegendary copy to Krark-Clan Ironworks for mana, which triggers the Nexus's replacement effect to exile itself and grant you an extra turn without sending it to the graveyard. Finally, Meticulous Excavation allows you to bounce the original Ugin's Nexus back to your hand, setting you up to recast it and repeat the cycle indefinitely.