Coming Down with Vorpentitis
As told in How to Cheat a Dragon's Curse, at some unknown time, the Doomfang came down with a disease known as Vorpentitis. The only cure for said disease is the potato, a vegetable that in the Barbaric Archipelago is known to Vikings as being legendary, as well as containing mystical properties. However, no one dared name the vegetable as it only grew in America, a land that only "crackpot weirdos" believed existed.
Sometime later, the then Hysteric Chief, Bigjob was near home at the Wrath of Thor from his voyage to America where he aquired a potato. The Doomang attempted to steal the potato to cure himself of Vorpentitis by attacking Bigjob, but failed. However he did manage to kill Bigjob. After this, for fifteen years, the Doomfang inhabited the Wrath of Thor without leaving. This caused no one to be able to enter or exit Hysteria for the time that he was there, thus making him a "curse" on the island.
Fifteen years after becoming the curse on Hysteria, Hiccup, Camicazi, Toothless, and One Eye arrived on Hysteria to take the potato to cure Fishlegs, who had aquired Vorpentitis. The group was able to steal the potato after coming into conflict with Bigjob's son, the current Hysteric Chief, Norbert the Nutjob. They sailed away in The Hopeful Puffin, but the Doomfang ambushed them and stole the potato. The Doomfang was then cured of the disease.
Becoming Hiccup's "Guardian Angel"
Many years later, when Hiccup was a young warrior sailing in his ship, it was followed by a Doomfang. Once in the Sullen Sea, Hiccup realized that it wasn't just any Doomfang, but the one he encountered near Hysteria many years before. The Doomfang became Hiccup's "guardian angel," helping Hiccup's ship "ever after" to the point that Hiccup lost count.
The Doomfang seems very hostile when his life is in danger, as he attacked Bigjob and Hiccup, trying to take the potato to cure himself of Vorpentitis. However, when Hiccup saved his life, he seemed very grateful to the point that he became his "guardian angel" and saved Hiccup "a hundred times over."
This Doomfang has a typical Doomfang appearance. Possessing only one head, he has a long neck, bright green eyes and teeth, a forked tongue, and a glossy black hide. He is described as very large and agile and possessing both wings and legs (though how many pairs of legs is never specified). His fire is blue and ice-cold.