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

What about the fact that Agrael can open portals to the Underworld? Only demons are capable of that! Besides, when he was exiled from Sheoh, demons continued to serve him. There's also another fact: Biara was also ready to betray the Overlord to help her lord, but when he underwent the Ritual of True Nature, she decided that things were going wrong, so she tried to steal the Griffin Heart. As for Xana, if Sareth had freed her father, she would have gained freedom by escaping from his body. So, what benefit would she gain from imprisoning Kha-Beleth?! Only love, I think...

Humans can summon demons; a typical example is Marabas, who is originally from the Academy, and the demons could have been either renegades or simply unaware of what they were doing; Agrael could have easily manipulated them. If Biara was ready 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 freedom; remember, when Kha-Beleth was released, he promised her eternal torment? She perfectly understood that by trying to bring Sareth over to her side, she was taking a risk, and that there was no turning back.