What about the lvl 400 dungeon @ snowhill? Can we have any custom lvl 400 dungeon with a few droprates such as requiem boxes, gm luck bags, 100k checks... just something for fun and to kill the lvl 400 boredom.
If you mean "The Witch's Dungeon" for Lv. 401+, it's a joke.
They made this joke at some point and snuck it in as like a "Coming never" message.
The reason that I can confirm that this is joke is because of the actual Witch's Dungeon. Yes, it is a map, and yes, it is in the game.
This is the map:
If you're an old player, you'd recognize this map. This is the map that you used to end up on if you made a new character.
You would arrive here, and go around and get the free items from the NPCs, then talk to Stan and be let off at the Gate of Coral Beach. Here, you would talk to the NPCs and do quests such as, kill a Torrobit, use your Beginner Skill, purchase Tangerines from Item Girl, and more. However, it was never removed from the game for some reason, and thus, is the Witch's Dungeon. The reason I do say "for some reason", is because despite this map not being removed, Gate of Coral Beach was removed, and trying to enter said area generates a soft lock.