How did Drago get the Alpha?

Question

I am confused about a scene in How To Train Your Dragon 2.

The "bad guy" says that in order to conquer the dragons, you need dragons and you cannot beat an Alpha without another Alpha (to challenge him). How the hell did he prison an Alpha? I mean the Alpha seems to be super strong.

I was wondering if someone could explain this to me or if it is just a plot-hole in the story.

Answer 1

Basically stolen from this answer on SciFi.se, which sources the official movie page, the answer is that he didn't imprison his Bewilderbeast, he raised it from childhood and conditioned it to be obedient to him.

Found as a hatchling during one of Drago Bludvist's earliest conquests, this particular Bewilderbeast suffered the misfortune of being raised by a madman.