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

Well, for me, the plot comes first...
No, the Overlord wouldn't have done that, and even Biara would have betrayed him and run away in fright. In Shi'o, children are only born to succubi and demonesses from other demons, but not from Kha-Beleth. Sareth was only needed by him as a savior; he didn't even consider him a son. Just a naive helper.