So, Hayate and the snake demon, Echiru, head into the demon world (the method of how they're going to do it escapes me at the moment...) and there they go to Echiru's "country", if you could call it that. The fertile females are forced to mate with fertile males to produce infertile warriors like Echiru, all controlled by a demon named Archfiend Verel. Verel is forcing the demons to create an army for him so that he can take over the human world and rule it with an iron fist. To this end, he created many demons called Fiends, which are bred only and ONLY to keep the Entah (the snake demons) in line.
After that, details get fuzzy...