The strength of a Lightning Breath ability is noted after its name (e.g. "Lightning Breath 20"). This indicates the attack's strength, and thus the maximum Damage it can cause (though this is subjected to To Hit rolls and the target's Defense rolls). Note that in-game, a unit's details panel will list Lightning Breath only as "Lightning" (followed by its strength), though the tooltip for this ability will show the full name.
Only a single unit possesses Lightning Breath by default: the Sky Drake. Its Lightning Breath attack is also exceptionally powerful, at Lightning Breath 20. Compounded with the Defense-reducing properties of Armor Piercing Damage, this is a very dangerous attack to almost any target.
Lightning Breath may not be added to any unit that does not possess it by default.
The Sky Drake possesses a unique ability that allows it to literally breathe out lightning bolts. These are concentrated towards a specific target, and can cause significant amounts of damage. Due to the nature of lightning, armor can do very little to stop it. Units attacked by Lightning Breath can, at best, try to dodge out of the way. A direct hit by this concentration of electricity is deadly to the vast majority of units.
Lightning Breath is automatically used whenever the unit voluntarily makes a Melee Attack against any opponent. Before the unit commences its actual attack, it will unleash a Breath Attack delivering a certain amount of Magical Damage, and includes an Armor Piercing Damage component.
This is carried out in a similar fashion to a regular attack. The Lightning Breath unit makes a number of To Hit rolls equal to the strength of the Lightning Breath attack. The target then gets to make Defense rolls to try and block some of this incoming damage.
As a Breath Attack, Lightning Breath occurs before any other exchange of damage or other attacks between the unit and its target. It precedes Gaze Attacks, Touch Attacks and any exchange of actual Melee Damage.
As a result, any figure in the enemy unit that is killed by the attack does not get to retaliate at all - it is already dead by the time its unit begins the retaliation (that is, if the unit survived at all). This confers a great deal of protection to the Lightning Breath unit: if it can kill a large number of enemy figures (or all of them) with the Lightning Breath, the target can return very little damage at it.
The Armor Piercing Damage that accompanies this attack is also extremely important: it halves the target's Defense score. In other words, the target may only make half as many Defense rolls as it normally does to try and block this damage, thus suffering (on average) twice as much damage from this attack as it would from other attacks. This means that the attack is effective even against well-armored targets, and absolutely deadly to low-armored targets.
Since Lightning Breath delivers Magical Damage, it can be completely blocked by a unit possessing Magic Immunity. Such a unit would have its Defense score boosted to 50 for purposes of blocking damage from the attack - and thus is likely to block all damage, even from the powerful Lightning Breath of the Sky Drake. The Armor Piercing Damage component of the attack does not help here, since it applies before the target's Magic Immunity is triggered.
Remember that the modifications to the target's Defense score apply only when it is blocking damage from the Lightning Breath attack. Once this specific attack ends, the target's Defense score is restored to normal.
Units with Default Lightning Breath Edit
There is exactly one unit in the game that possesses Lightning Breath:
Sky Drake - Lightning Breath 20
With a Lightning Breath of strength 20, the Sky Drake delivers an extremely powerful attack that can kill enemy units before they get any chance to retaliate. Given the Armor Piercing Damage caused by the attack, even heavily-armored units are at risk from it.
Acquiring Lightning Breath Edit
There is no known method of adding a Lightning Breath ability to any unit that does not already possess it, nor to permanently increase a Sky Drake's Lightning Breath strength beyond its default.
Improving Lightning Breath Edit
Sorcery Nodes bestow a +2 bonus to many of the properties of any Sorcery Fantastic Creature fighting in their area of influence. This bonus applies to the Sky Drake, improving many of its abilities by +2, including Lightning Breath. It's a small bonus, but is nonetheless welcome.
This bonus is transient: it applies only while the Sky Drake is doing combat within the vicinity of the Node. It is not applied when doing combat anywhere else.