Dragon breath

Dragon breath, or breath weapon, was a term used to refer to a special ability that was inherent to dragons and the majority of dragonkind, even those who merely had draconic ancestry.

Nature of Breath
The “breath” aspect part of "dragon breath" was a bit of a misnomer, as it was not discharged from the lungs through exhalation. Rather, its stream of energy was more accurately vomited.

Depending upon the type of dragonkind or draconic ancestry, a breath weapon had different properties. There were nearly as many varieties of dragon breath as there were types of dragons. Though a number of dragons possessed the same type of breath and some possessed two distinct breaths.

Biological
In true dragons, the elemental or magical energies that granted a dragon their breath weapon were said to be generated by the heart and suffused throughout their body within the blood. The source was previously thought to be a large blood vessel called the fundamentum, or draconis fundamentum, but that merely channelled energy from the heart to a chamber in the gizzard. This organ stored a dragon's magical energies until they were needed to be unleashed as dragon breath.

Some breath types required an instrinic temperature, which was maintained internally by the elemental or magical energy that flowed through a dragon's bloodstream. While other breath types, such as acid, didn't have any such necessities. Additionally, dragon eggs were inherently resistant to harm from the breath weapon of their parents, though this resistance waned the closer they came to hatching.

Magic
Dragon breath had an inherently magical nature to it. Some believed that the magic permeating the Prime Material plane empowered it. Others believed that the different species of chromatics, as well as metallics, were each imbued with elemental magic and their breath weapon was a manifestation of that elemental energy.

Sometimes, a dragon's breath could be affected by the magic that permeated within their lairs. Such as clinging to and lingering on those afflicted by it or recharging at an incredible rate.

With sufficient practice, some dragons were capable of shaping their breath weapon into a bipedal, elemental creature known as an animated breath.

History
On Toril, some scholars from Candlekeep proposed the proto-dragon theory, claiming that the dragons of their world gradually developed breath weapons through evolution. This theory purported that a common ancestor, ignidraco, developed a digestive tract that allowed them to regurgitate their own intestinal gas and ignite it with an enzymatic, hypergolic reaction in the back of their throats. The proto-dragon theory went on to claim that the ancestors of green dragons and black dragons developed their breath weapons from further modifying their digestive tracts.

There were many detractors to the proto-dragon theory, who pointed out that it could not fully explain the myraid of other elemental and non-elemental breath weapons that dragons possessed.

True Dragons

 * Almost every kind of dragon, including wyrms and faerie dragons had this innate power and ability.
 * There were a number of chromatic and metallic species of dragons that had similar types of dragon breath.
 * Crossbreed dragons, those born through the mating of two different species, could potentially develop a breathe weapon that was a combination of both their parents'.
 * A dzalmus had a breath weapon that was a life-draining gas, though only one of their three heads could unleash it at a time.
 * Ethereal dragons had a breath weapon that was a cone of pure force, which uniquely could affect even creatures on the Ethereal plane.
 * Rust dragons possessed two breath weapons, one that was acid and another that was an extremely corrosive liquid.
 * Styx dragons possessed two breath weapons, one that was acid and another that was an intelligence draining gas.

Lesser Dragons

 * Dread linnorms, a two-headed species of linnorm, had a distinct breath weapon for each of its heads. One breathed fire, the other a freezing wind.
 * Stygian linnorms uniquely had a breath weapon that was a mixture of acid and water from the River Styx.

Dragonkind

 * Most dragonborn were capable of exuding some type of dragon breath. The type of breath weapon was dependent upon what chromatic, metallic, or gem draconic ancestry the dragonborn had. And some believed that, like dragons, dragonborn were imbued with elemental magic. Some dragonborn whom believed this would claim that the dragon deity Io had gifted them that power as a sign of his favor. Regardless of its origin, it was typically as powerful as that of a wyrmling of the same type.

Magic

 * Rarely, weapons could be magically altered or enchanted to produce dragon breath.

Undead

 * Dragons resurrected as lesser forms of undead dragons lost their breath weapons, such as skeletons and zombies, though some sources claimed that zombies retained a weaker version of their breath weapon.
 * Dracolichs retained the breath they had in life, but were required to eat in order to refuel it.
 * The ghosts of dragons had their own unique form of breath that was a cone of gray mist, which sometimes took the form of a ghostly parody of the breath they had in life.
 * Hoarder dragons retained the type of breath they had while alive, but with an added spray of pelting coins