My memory is probably playing tricks on me, but I could've sworn that there was mention of both Mannanon (or however it's spelled) and Mordack having been working together in KQ5. Something about them being brothers or friends or something.
However, I may also be pulling information from my memories of the Companion's story section, which was really interested and in ways helped me enjoy the experience of playing the games even more. I often would read the stories as I played the games during later replays, and always after I'd finish a game I'd read the book's version of events. I may be thinking of the mention but it was only in the Companion. Or maybe as you say, I'm getting my memories of 5 and 6 a little mixed up.
Age has not helped my memory this last year..