Breath attack is a special ability typical for living dragons, but also for Firebirds and Phoenixes. It enables them to attack two consecutive hexes at once, making individual damage to both hexes. However, creatures that are two hexes wide do not suffer double damage from the breath attack. Although creatures with breath attack specialty cannot directly attack allied creatures, they can - similar to Liches' Death Cloud and Magogs' Fireball Attack - cause damage to allied creatures located on the second hex. In other words, breath attack will also damage the hero's own troops if they are standing directly behind the attacked hex.
Damage for each attacked stack is calculated separately. Black Dragons' hate against Titans works even if the latter were not attacked directly and suffered from breath attack. If your Black Dragons by mistake attacked Titans who are part of your army, they will still deal 150% damage.
Bone Dragons, Ghost Dragons , Faerie Dragons and Crystal Dragons don't have breath attack.
Creatures with breath attack: