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
Sir Alonne from Dark Soul 2.
Honourable mention to the leader of the trolls in Pathfinder: Kingmaker.
Run back to that boss is awful. Worst in the series
It’s like they took the complaints about iron keep NPC placement as a challenge.