Draconis, from Baldur's Gate II: Throne of Bhaal.
Mother.
Fucking.
Draconis.
For the record, Baldur's Gate II is probably my favourite game of all time, and the expansion was mostly very good, with Watcher's Keep being the best part of it: it's the best dungeon in the series, and one of the best dungeons I've played. Unfortunately, the main story of the expansion suffered from serious difficulty spikes that, given the nature of your character by this stage, are hard to believe in the context of the story, and simply frustrating to play.
Draconis is the worst of these. His function in the plot is nothing; he is simply a door guard for his father. However, I couldn't beat him. I still haven't. You meet him alone outside a fortress, so you assume he's going to be pretty powerful. And he is: he's a high-level mage, but nothing your party can't handle by this point. Then, when you kill him, he says "Did you honestly think I'd be so easy to kill?", and transforms into a DRAGON.
Now, by this point, I'd killed a number of dragons. However, by the time he transforms, most of the buffs on my party had worn off, and he'd killed my summoned creatures while in mage form. On later attempts, I would wait until after the transformation before summoning them, whereupon he cast an instant death spell on them, putting me back where I started. As a result, the following battle essentially involved him curbstomping my entire party over and over again. He does things that are impossible by the game's rules; one of the most annoying is that he can still cast certain spells while your mage has stopped time.
After several dozen TPKs, and much completion of side quests in pursuit of more levels and better gear, I gave up.
