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

Actually, Railag doesn't stand a chance against Saref. This "youngster with a couple of tricks" defeated the "Messiah of Ash," who had a skull, and Aranthir is much more powerful than Railag. Also, Kha-Belekh could have provided information to Saref; he suspected the betrayal in advance during the campaign and could only destroy him because his subordinate, Vaier, was a fool. Biara may be strong, but she values her life. And then, a group of heroes gathered.

He didn't even recognize his servant without the crimson armor and underestimated Agrail in the fight against him.

He recognized him; in one of the videos, he spoke with him, and he addressed him as Agrail, not Railag.