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When an opponent casts a spell, sacrifice Lunar Force and counter that spell.
The spell will be countered even if Lunar Force can’t be sacrificed, most likely because it has already left the battlefield. Casting a spell in response to an opponent’s Lunar Force’s triggered ability is generally a bad idea.
You can’t choose not to sacrifice Lunar Force.
You sacrifice Lunar Force even if the spell’s already been countered, perhaps by the ability of another Lunar Force, or if the spell can’t be countered.
