Lord Soth, from Death Knights of Krynn (old SSI AD&D game).
Goddamn that bastard. Goddamn him.
For one, he's a freakin' Death Knight (instead of some undead fragile wizard, this guy was an engine of destruction before becoming a lich). Insta-fireballs whenever he wants, causes fear, immune to almost all damage ... and the prick's brought along about 12 of his buddies who are all varying shades of immovable object/unstoppable force.
Seriously, I have never had a moment in a game where I've felt more like a bunch of nerds having the absolute bejeezus beaten out of them than that final fight against Lord Soth.
It was like watching 6 jawas fight 12 Darth Vaders (Vader I'm sure being the inspiration for Lord Soth).
The fight would invariably start the same way - Lord Soth and a half dozen of his lackeys would open the fight with 6 delayed blast fireballs straight into the middle of the party. Even if you managed to save vs. spell, you still took half damage (long time no play, but I think a fireball from a Death Knight would do about 8d6 damage).
It was simply luck whether you even had one person left standing after the first round, considering one half-decent roll on damage was all they needed to incapacitate all of your party. And they got six attempts. Before your Knight of Solamnia even drew his freakin' sword and yelled 'CHARGE!'