Dragonborn

Dragonborn (竜天成, Ryūtensei) is the curse or after effect inflicted upon a Dragon Slayer due to prolonged usage of Dragon Slayer Magic.

Description
Dragonborn is the term coined to any individual —such as Jiretto— who was not so fortune enough to have developed antibodies against a Dragon-esque transformation, and with proper training, has come to possess a degree of control over it. Indeed, Jiretto is capable of transforming aspects of his physical likeness to portray that of an actual Dragon; much like the notorious Dragon King himself, Acnologia, Jiretto is even capable of a complete Dragonic Transformation, increasing his size tenfold so as to allow him to compete with the titans of the world. It would also not be farfetched for one to come upon the assumption that Jiretto's human form is no longer his true identity; for he is now but a Dragon, one who has instead taken to the courses of impersonating a human.

Achieving
When a mere human elopes himself in the vast tranquilities of taming the might of a Dragon, horrendous consequences are to take place without question. For, this is ultimately the cost of harboring such power on a consistent basis. Naturally, the preliminary stage incorporated within the elements of conceiving this breed of Slayer Magic is none other than intent; the human scarifies himself to the Dragon in a quest to obtain greater power, knowing all too well that such a route would eventually lead to their swift demise, through the form of an unauthorized transformation.

The birth of a Dragon is a truly extraordinary sight to behold; yet, humans alike resent this physical alteration, coming, even so, to regret ever having ventured into the depths of Dragon Slaying Magic. However, modern circumstances would instead foretell the blasphemy of Dragons themselves willingly complying to act as a tribute in the place of their beloved "children"; such an act develops antibodies within the human, annihilating the possibility of ever reaching the heights of a true Dragon. Nowadays, the only such transformations that take place come with those whom are deemed of being apart of the "Second Generation", as they instead received their Magic through a Lacrima, and thus, lacked any forefather who would have sacrificed himself to develop antibodies. A prime example of this context is Erik, none other than the famed "Cobra" of the Oracįon Seis, whose hands and arms turn to claws and scales, respectively, when his Dragon Slayer Magic comes into play.