Getting some inspiration from (Eroninja)
Valka and Hiccup are taken by Cloundjumper, later hooking-up with each other.
They start to capture females, if only girls from Httyd or not is up to you, and enslave them to make a society for themselves, slaves incuded, and their dragons. The method(s) they use is/are free game.
If you want to try it put the title in the comments, so others can check it out.