Attack together with nearby backup allies for extra damage.
Initiating a Chain Attack
A chain attack can be initiated by attacking with a unit when another allied unit(s) that is in a Backup class is also in attack range of the same enemy. Similar to Three Houses linked attacks, whether or not an enemy is "in attacking range" of an ally depends on their currently equipped weapon. This can be manipulated by equipping a longer range weapon before initiating the chain attack, or by trade equipping (if the allied unit has already moved).
When a chain attack happens, the participanting units will always attack first. The damage dealt by a participant unit does not depend on stats or equipped weapon at all, instead it always deals 10% of the enemy's maximum HP rounded down, and will always deal at least 1 damage. A chain attack always has 80 displayed hit and 0 crit.
The initiating unit always gains the same amount of experience that they normally would have without a chain attack, even if a participant attack kills the enemy without the initator getting a hit off. Participanting units do not gain any experience from helping out.Some skills can affect the eligibility of chain attack participants, for example Emblem Lucina's sync skill Dual Strike.