Cast Alchemist's Gambit for its cleave cost by paying {4}{U}{U}{R}.
Riku's first ability triggers - choose not to pay.
Holding priority, cast Narset's Reversal by paying {U}{U}, targeting Alchemist's Gambit.
Riku's first ability triggers, causing you to pay {U}{R} to copy Narset's Reversal, with the copy targeting the original Narset's Reversal.
Resolve the copy of Narset's Reversal, copying the original Narset's Reversal targeting Alchemist's Gambit, and returning the original Narset's Reversal to your hand.
Resolve the copy of Narset's Reversal created in step 5, copying Alchemist's Gambit and returning the original Alchemist's Gambit to your hand.
Resolve Alchemist's Gambit, causing you to take an extra turn after this one.
Repeat each turn.

Infinite turns

Lock

Riku of Two Reflections

Narset's Reversal

Alchemist's Gambit

Declare Fire Lord Azula as an attacker, triggering its firebending ability, adding {R}{R}.
Holding priority, cast Alchemist's Gambit by paying its cleave cost.
Fire Lord Azula's second ability triggers, copying Alchemist's Gambit.
Holding priority, cast Narset's Reversal by paying {U}{U}, targeting the original Alchemist's Gambit spell.
Fire Lord Azula's second ability triggers, copying Narset's Reversal, targeting the original Narset's Reversal.
Resolve the copy of Narset's Reversal, causing you to copy Narset's Reversal targeting the original Alchemist's Gambit spell, and then return the original Narset's Reversal to your hand.
Resolve the copy of Narset's Reversal, copying Alchemist's Gambit and returning the original Alchemist's Gambit to your hand.
Resolve both copies of Alchemist's Gambit from steps 3 and 7, causing you to take two extra turns after this one.
Repeat during your second extra turn.

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