Azrael the cat
Arcane
The fight at the end of the 'grand ultimate' ending for ADOM. Granted, to even get to it your character has to be so insanely absurdly stupidly powerful not just due to the regular game's difficulty, but also due to the insane requirements that have forced you to go almost all the way to one alignment and then go all the way to become a champion of the opposite alignment, do about 4 extra uber-boss fights, extra areas and kill pretty much everyone that's a power in the game. So you think that because your little ASCII guy is powerful enough to cross into the chaos plane, that's there's nothing that can possibly threaten his over powered ass.
Then about 5 turns later you realise you were very very fucking mistaken.
Then about 5 turns later you realise you were very very fucking mistaken.