How is Dark Messiah connected to Heroes of Might and Magic?

Humans can summon demons; Marabas is a typical example, and he actually comes from the Academy. The demons could have been either renegades or simply unaware of what they were doing, and Agrael could have easily manipulated them. If Biara was willing to betray the Overlord, it was only to take his place. She was the one who informed on Agrael and kidnapped Isobel. Xana wouldn't have gained her freedom; remember, when Kha-Beleth was released, he promised her eternal torment? She understood perfectly well that by trying to bring Sareth over to her side, she was taking a risk, and that there was no turning back.
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Agrael himself said in the game that they were loyal to him out of fear, and that they were perfectly aware that the Overlord had banished him. Biara may have informed on Agrael, but he wasn't banished solely because of that. Kha-Beleth himself told him that he had known about his ambitions for a long time. I don't remember a moment in the game when Kha-Beleth promised Xana torment. My game is glitching: it doesn't show any cutscenes at all, so I had to watch it on YouTube in English, although I still didn't fully understand it. Tell me the gist of it, or better yet, if you can, give me a link to a good video in Russian...