If you haven't already, look at the 5E Mystara players guide. I'll attach a link. It's got most of the important information on the setting in one source combining all of their timelines with a look at every single one of the major nations. I'm the one that wrote it, I'm very proud of it, but I...
Voyage of the Princess Ark. Was written by the line editor, later a good chunk of it was compiled in Champions of Mystara. A good chunk of it was retconned with the release of Red steel
Okay this is kind of my specialty. First off what edition of the game are you trying to run?
The 2nd edition Mystara books were unfortunately riddled with mechanical issues. If you want to go back to BECMI You can always start with the Gazetteer.
If you want information on the lore then you...
It was deliberately made as a mystery bad guy. There's no stats for it, there's no art for it, and the physical description has at least six different versions. It's incredibly powerful, messes with memory, and most of what we know about it's background and motivations are deliberately left...
We played the audio CD for Mark of Amber to a French speaker once. I think he started developing hives because of just how bad the accents were. Which is really sad because the original line editor for Mystara line editor was a fluent French speaker