Question about the plot.

The chronological order of events in the fifth installment goes as follows: Heroes of Might and Magic V - Hammers of Fate - Tribes of the East - Dark Messiah of Might and Magic. In the latter game, you play as Sareth, the son of Kha-Beleth and Isabel, who is destined to free his father so that he can bring good and joy to the world.

So, here's the question... how was Sareth even conceived? How did Kha-Beleth and Isabel even have a relationship?