When an opponent casts a spell, sacrifice Lunar Force and counter that spell.
7/13/2016 You can’t choose not to sacrifice Lunar Force.
7/13/2016 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.
7/13/2016 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.