Fallen Knight

There should be some kind of story behind all this. There's this uber-demon sitting around, and his domains are directly connected to Mythgard. He's called Chaos, very grandly. Who he is, Kha-Beleth knows! He just sits there, essentially preventing you from entering the local library, where a powerful Necromancer has decided to hang out. In another place, a mage, even more powerful, is relaxing. He just lives with a huge army nearby, next to the Empire. As if he's guarding the border. Who he is, why, what's his purpose – who knows about these mages! And finally, there's a dwarf. Redhaven had a powerful dwarf guarding the Rune of Fire; this is his brother, guarding another Rune of Fire. That's it. Also, there are supposedly Necromancers of Erish, but for some reason, they have a demon flag, and the epic description of the Necromancer invasion from the map preview isn't reflected in the story at all. The only one who actually controls cities here is Hafaad.

Added 2 minutes ago
If you play the battle with Chaos from a save again, it will most likely break even more, and you will win immediately or shortly after killing the real Chaos demons, skipping most of the summons. There's this quirk. I've encountered this bug myself and passed it by playing the battle again.