Narset's Reversal + Isochron Scepter

Narset's Reversal

Isochron Scepter

How it works

1. Cast the extra turn spell by paying its mana cost.
2. Holding priority, activate Isochron Scepter by paying 2 and tapping it, casting a copy of Narset's Reversal.
3. Resolve the copy of Narset's Reversal, causing you to copy the extra turn spell, and return the original extra turn spell to your hand.
4. Resolve the copy of the extra turn spell, causing you to take at least one extra turn after this one.
5. Repeat each turn.

This elegant engine achieves infinite turns by looping a single extra turn spell right back to your hand every single round. It all comes together thanks to Isochron Scepter, which quietly stands ready on the battlefield to repeatedly cast copies of Narset's Reversal for a mere two mana. By using Narset's Reversal to bounce the original extra turn spell back to your hand while simultaneously creating a copy of it to resolve first, you ensure the spell is never lost to the graveyard. Instead, you get to take your extra turn, keep the original spell safely in hand to replay next turn, and repeat the entire sequence indefinitely.