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
Margit and godrick were my first two souls bosses. I was super under-leveled when I tried them. It took probably 30 tries for each? It was terrible.
Really though though the worst one was a boss from archvale. I played the game on "hard" without realizing that it was actually supposed to say "hardcore." I've never played a game that I couldn't beat "hard" in so I tried it. I couldn't understand why I couldn't beat the boss. I just fucking couldn't beat it. I got mad and slapped myself in the face. It gave me a concussion that has kept me out of work for 6 months now. I just tell everyone that I bumped my head on a cabinet. Turns out that hard mode is something that people with thousands of hours in bullet hell games struggle with. That was one of my first bullet hell games, I had like maybe 40 hours total in those kinds of games.
Anyway I don't self harm anymore because holy shit the consequences of my actions. Well I do very rarely but I hit my thigh instead which is a lot safer. lmao this happened like 2 months after breaking off the inside door handle of my car on accident. I was just trying to open it but it was locked. It just kinda came off. :(