A boss you died to 1000 times, an enemy that still strikes fear into your heart every time you replay your favorite game, etc.
For me it's the Blood Starved Beast from Bloodborne. I died to it probably over 100 times my first playthrough no matter how many times I replay the game I still get torn to shreds repeatedly. Honorable mention cazadors from New Vegas
I don't know what the hell the deal is but Ceaseless Discharge from DS1 fucks with me endlessly. I can never get the kite strategy to work and end up just having to chug through the damage. I've played dark souls like a million times too and have no idea what the deal is. I love Bed of Chaos compared to Ceaseless.
There's people who fight Ceaseless Discharge?
Do you also fight Yhorm without the Storm Ruler?
Did you loot the corpse by his spawn? I think he only chases you to the ledge if you steal his dead sister's clothes.
That is probably it actually. I bet that explains some of it anyway.