Experimentation

(Takes place after HTTYD1)
It was a seemingly normal day in Berk; vikings doing Viking things, dragons doing dragon things. But what no one was expecting was for an army of dragon trappers to attack, kidnapping the weak and vulnerable while slaughtering anyone who got in their way.
Hiccup, still not used to his prosthetic, was lucky enough to fall under the 'weak' category. Along with a few others of his tribe, he is taken to the base of Viggo Grimborne and chosen to be a part of the next group of test subjects for a gruesome experimentation. A mixture, or potion if you will, made up of many ingredients from dragons of all kinds. The main and most important ingredient: diluted changewing acid.
When this mix is injected into the veins of a human an incredibly disturbing fate is sealed upon them. They will be slowly transformed into a dragon.