The Sixth Doctor + Karn's Temporal Sundering + Isochron Scepter + Unsubstantiate

The Sixth Doctor

Karn's Temporal Sundering

Isochron Scepter

Unsubstantiate

How it works

1. Cast Karn's Temporal Sundering by paying 4.
2. The Sixth Doctor triggers, copying Karn's Temporal Sundering.
3. Resolve the copy of Karn's Temporal Sundering, causing you to take an extra turn after this one.
4. Holding priority, activate Isochron Scepter by paying 2 and tapping it, casting a copy of Unsubstantiate without paying its mana cost.
5. Resolve Unsubstantiate, returning the original Karn's Temporal Sundering to your hand.
6. Repeat each turn.

This brilliant extra-turn loop relies on the way legendary permanents and exile effects interact with copying spells. When you cast Karn's Temporal Sundering, The Sixth Doctor triggers to make a nonlegendary copy that grants you an extra turn while the original sits on the stack. Before that original resolves, you use Isochron Scepter—which has previously imprinted Unsubstantiate—to cast a free copy of Unsubstantiate targeting that very same original Karn's Temporal Sundering. This bounces the original back to your hand so you can recast it on your next turn, leaving the exiled card safely away from Karn's Temporal Sundering's own exile clause.