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

Well, Xana resurrected Sareth not out of love, but for her own benefit. Without him, she is unable to claim leadership among the demons. A mere whelp wouldn't be able to take the skull from Arantir, and she wouldn't be able to seal the Master herself. As for the video... it was actually Biara, and that's a well-known fact. Isabel was completely unaware of the events in "her lands" at the time, while Biara was ruling there (and unfortunately, she was defeated).