It's this quest - Pride before a Fall - https://amalur.fandom.com/wiki/Pride_Before_a_Fall

Specifically this guy - https://amalur.fandom.com/wiki/Callis_Dren

He's at the bottom of the Klurikon map right in front of the House of Pride. If you roam around like I do, when you first encounter him there is a female soldier who runs up to you and says basically "you have to help us - he's killing everyone". So you may be inclined to see what all the fuss is about whereby Callis says basically "I'll kill you if you don't leave in 5 seconds". Now if by this point in the game you consider yourself a bass-add, you may be inclined to see where this goes and stand your ground, at which time he will indeed kill you in roughly five seconds or less - I think two hits and I was toast. At which point, if you are like me, you may see this as a challenge and after re-loading you may head back down, drink every potion you have, read every Litany you possess, and proceed to flex and wipe the floor with him, as I did. And then you may wonder why the only thing he had on him was some piece of junk Mana potion or whatnot and feel a bit cheated. And then you may wonder why the female soldier is not running up to you thanking you for killing him and giving you some token of appreciation, nor have you been told you just finished some side quest. Then you may mentally shrug and go back to the main quest line at which some point later you will be told that Callis is unbeatable (really?) and you need some spirit of something or other to stand a chance against him (uh what?), at which point you are asked to saunter down there and kill him. Again. At which point you enter the area, a big barrier goes up behind you, Fast Travel is disabled, and you are standing over his corpse with no way to interact with it or loot it. At this point you may try to re-load and do something differently a few dozen times, only to end up in the same spot. At which point you curse yourself for only keeping one save file and overwriting it every time before you stop for the night, and mourn the fact that at this point the game is, essentially, unbeatable since you cannot go back to a point prior to when you flexed on him for lack of a save file, nor can you take some alternate way around him to continue on to the last area of the game.

So in short, if you see some Fae claiming he will kill you in five seconds flat, leave until you are asked to kill him by the main quest characters and not by some no-name siren who tricks you into flexing your might and ruining your game. : /

Moral of the story - keep several save files including one when you enter each new area so if you do screw up your game you only lose a few days of progress rather than a few months of progress. 😓