Breath is a unique transcendental skill used by members of the Vritra clan, i.e. both the nastikas and their half children. The exact effects of this skill vary depending on the user. Half children have the same type of Breath as their nastika parent. Taksaka and Kasak use a form known as Destructive Breath.
Breath does not necessarily need to be fired from a dragon's mouth, but it is more powerful and quicker that way. Kasak has been shown firing at Maruna using his hand while his mouth was still clamped shut.