The Lost Halls Key open Lost Halls dungeon.
The Lost Halls is an endgame dungeon designed by Toastrz and Kiddforce. The original design document was made by Toastrz but the final version contained some noticeable differences and was “grander in scope.” The Records of the Lost Halls are a series of official lore entries that give backstory into the dungeon and hint at dungeon mechanics. The dungeon was released on August 1, 2017, coinciding with Month of the Mad God.
In addition to the main dungeon, there are 2 secret areas, each with their own boss. The dungeon is a source of numerous stat potions, top-tier gear, and a large number of powerful Untiered items.
The portal has a chance of dropping from the Lost Sentry event boss, which spawns once per realm.
The current dungeon is more commonly known as “Lost Halls 2.0.” The “original” version of the dungeon (before Patch X.26.0.0) can be found here. The dungeon rework was due to several complaints that the main dungeon was too boring and tedious to clear, and often required large groups (30+ players) to complete in a timely manner. The update changed most enemy and boss behaviors, increased room variety, redid most of the artwork, buffed several Lost Halls UTs, and added HP scaling to all enemies to make the dungeon viable for most group sizes.