A Critical is an attack that deals more damage to enemies at random. The probably of a character dealing a critical attack is influenced by a number of different factors such as their Skill, their equipped weapon, their supports, and the enemy's Luck.
In earlier games, a Critical will double the attacking character's Power, but from Fire Emblem: Fūin no Tsurugi onwards, Critical hits will triple the amount of damage normally dealt to the enemy. That being said, if your attack does no damage in the first place, a critical hit will also deal no damage.
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