To crack a mirror is to provide an opening for the reflection to escape
Monster Manual
The interactions of the cards reveal numerous strategies and a near infinite variety of working cities
Set: Commander Anthology Type: Legendary Creature — Human Cleric Rarity: Mythic Cost: {1}{R}{W}{B} Flying
Clear, Fair Magic [Mystery Booster 2 Playtest Cards] Flesh and Blood To crack a mirror isSet: Mystery Booster 2 Playtest Cards Type: Instant Rarity: Rare Cost: {1}{G} Split second (As long as this spell is on the stack, players can't cast spells or activate abilities that aren't mana abilities.) Destroy target artifact, enchantment, token, emblem, day night tracker, monarch, initiative, dungeon, city's blessing, Attraction, Contraption, plane, scheme, vanguard, bounty, conspiracy, elite creature, or Magic: The Gathering Online avatar.