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

Um, I guess it's because Aranthir killed Sareth...
Aranthir isn't a Watcher; he just wants to save Ashan because Ashan told him to, and Shio doesn't have any Watchers. It's just a parallel universe created by Sar-Ilum to imprison the Overlord forever. However, the spell didn't work as intended...