Walkthrough tips, Discussion of Campaigns [Heroes 5]

The campaigns in Heroes 5 are indeed quite short (only 5 missions for each faction), but they are interesting. The missions differ from each other and don't seem repetitive; one mission might give you many tasks. I've already completed the human campaign (it was easy) and the demon campaign (I struggled in the third mission). Now I'm playing the undead campaign (currently on the second mission), but I don't understand why the mages are attacking the humans. They seemed to be more or less friends...