The quest where you sneak through Eldieme to the island off the coast of Batallia. My friend and I did this at the minimum possible level in the original game. All of the endgame zones were terrifying then.
They really were terrifying...
On that note.
Waaaay back in the day, early 2000s there was an event quest that would lock you at lvl 1 with sneak/invisible and sent you into the most dangerous zones in the game at the time and had you search for ??? Spots to get a prize.
I learned to fear and respect Den of Rancor after that event.