A bit out there: Tir Na Nog aka Another World from the MMORPG Mabinogi.
Back when the game was first released, there wasn't a story freely available so most people never reached this area. Eventually the story was released for free so more people could enter it. There were a number of conditions to get in, but the most memorable for me was that it had to be a Saturday. Someone needed to provide a special pass for a dungeon to get in. At the time the dungeon was inside a mining town that was I believe at the farthest point from the starting town, and since mounts cost real money, it took a bit of a trek to even get there.
Once you completed the dungeon you would enter a portal that took you to Tir Na Nog, another world, which was essentially a dark world version of the starting area of Tir Chonaill. There were only few NPCs in the area that sold some medical supplies like bandages. It was essentially a ghost town version of the peaceful starting area, with zombies inhabiting the nearby cemetary. Most of the enemies both inside and outside of the dungeon could one hit kill just about anyone in the game. The entire time you were there was essentially a race to complete the dungeon against dwindling resources, as you couldn't purchase things to revive party members and you couldn't repair equipment. If you screwed up too much you'd have to try again a week later after restocking supplies.
The goal was to reach and complete "Albey Dungeon", which housed Glas Ghaibleann, the final boss for one of the early story archs. The dungeon was in the same location as the easiest dungeon in the game. It was filled with monsters that could pretty much one hit kill you, especially Gargoyles which were extremely fast and had a decent amount of HP. Glas Ghaibleann at the time was actually somewhat difficult; his health regened too fast to harm him conventionally, you needed to wound him to deal permanent damage. The only way to do that was with: a) an Archer, which would require a crap-ton of arrows and most people didn't have high enough skill levels to make that efficient, or b) to kill the gargoyles surrounding the boss. Once you killed them you needed to burn passes they held which would make you temporarily invulnerable and deal all wound damage, so you could progress against Glas.
At the time it was one of the most difficult dungeons in the game, so I would spend time with friends sitting on the dungeon entrance and helping people complete the dungeon. People wanted to finish it to unlock a special transformation that they would be able to do once per day, which made them much more powerful.
The area was also known as a great area to level up a skill called "Windmill", which was one of the best skills in the game for any player. People would windmill zombies in the cemetary for hours, which were extremely slow but also very strong and had a lot of HP.
Its mostly my favorite area because of all that nostalgia, but also because the dark setting was a strong contrast to the relatively upbeat atmosphere throughout the rest of the MMO. Unfortunately its hard to capture that in pictures.
Doorway to Another World
Tir Na Nog (Basically only picture I can find)
Albey Dungeon entrance:
Glas Ghaibleann
