ROBERT SCHWALB (Designer of Exemplars of Evil) from WotC's Product Spotlight, September 14, 2007 at http://www.wizards.com/default.asp?x=dnd/4spot/20070914a.
Here's the Mystara-relevant part of the Wizards interview:
Wizards: Although all of the exemplars are your children in a sense, do you have any favorites among them? (And for that matter, any favorite or memorable villains of your own or others’ creation from games you’ve played?)
Robert: Oh dear, must I choose?
[snipped description of a non-Mystaran villain]
Outside of Exemplars, I’ve played D&D for over twenty years and in that time I’ve faced, fought and designed a number of wicked characters, many of whom are still near and dear to my heart. Looking back, it’s hard to pick one from the rest, but since the gun’s to my head so to speak, I’d have to say Cathandramus. He’s an interesting case for a number of reasons. Cathandramus is actually a misspelling of Catharandamus from the classic Palace of the Silver Princess. He makes an appearance in the Tome of Magic too, by the way. Coincidence? He was also an accident. I never intended for him to become a principle antagonist in the campaign; he just wound up that way.
The bold heroes were investigating the queer ruins of the Palace. The encounter with this villain was cool, memorable, and led to Catharandamus escaping with his life intact. Since he lived, a rarity for many villains, I jotted down his name—incorrectly—so I could use him later. When the opportunity presented itself to reveal him again, I, on the fly, recast him as a cultist of Asmodeus, who was a principal adversary in that massive campaign (some 30 players, two to three groups, and spanning almost a decade of gaming). My villain managed to escape again, this time bloodying the PCs’ noses. Over the months and years to come, he would make several appearances, each time escaping, each time stoking the fires of hate within my players. Cathandramus grew with the heroes, evolving with the campaign until he finally set aside his mortality to become a dreadful lich. Cathandramus was responsible for the spread of a vile cult, an instigator of numerous wars, corruptor of allied NPCs and very nearly brought the world to ruins with his sinister plots and wicked machinations. When the player characters finally confronted and defeated him, it was one of the signature moments in the campaign and one they still talk about almost an entire edition later. So, yeah, Cathandramus. He’s my boy.