The Sixth Doctor + Ugin's Nexus + Vedalken Mastermind + Krark-Clan Ironworks

The Sixth Doctor

Ugin's Nexus

Vedalken Mastermind

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 Vedalken Mastermind by paying and tapping it, returning the nontoken Ugin's Nexus to your hand.
5. Activate Krark-Clan Ironworks by sacrificing the Ugin's Nexus token, adding CC.
6. The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
7. Repeat each turn.

This clever loop revolves around casting Ugin's Nexus and letting The Sixth Doctor duplicate it so you have both a real copy and a token copy in play. You then use Vedalken Mastermind to bounce your original Ugin's Nexus back to your hand for re-casting, while Krark-Clan Ironworks sacrifices the token to generate the mana you need to keep going. Because the token is sacrificed, Ugin's Nexus’s replacement effect exiles it and rewards you with an extra turn, bypassing its own restriction against taking extra turns since you are generating them through the token's exile rather than a standard spell or ability.