This page is covering the unused area in Super Mario 64 DS. Are you looking for the Bowser Room, a similar anomaly in the original Super Mario 64?
A supposed image of Bowser's Chamber.
Bowser's Chamber is an unused area in an early build of Super Mario 64 DS, theorized to have been shown off at E3 in 2004. It is a small, rectangular room with gray flooring, a brown lining around the perimeter of the floor, and green walls similar to that of the basement. It would contain a boss fight against Bowser using all 4 player characters. Whether or not this was going to be part of the main game, or the multiplayer mode is unknown. However, it has been reported to appear in the game due to personalization.
Description[]
Bowser's Chamber is typically found in a hidden area of the castle such as the Black Box, and usually drops you into the room seen in the screenshots, but has also been known to drop you in an area leading up to the boss fight, commonly described as either a maze or a level in the same vein as to a Bowser level, usually as a hidden area inside of Princess Peach's Castle or in other anomalous areas such as Bowser's Castle.
The boss fight with Bowser himself is usually different from regular Bowser battles, as there are no bombs in the arena like those in the 3 in-game Bowser boss fights. Some players have found ways to damage Bowser with each of the 4 characters. Bowser can occasionally throw Bob-ombs that Yoshi has to throw back, similarly to the Bowser fight associated with the Halved Key anomaly. Mario can throw Bowser into a wall to knock them, and then other players can ground pound them to damage and defeat Bowser.
Rewards for completing this level varies to dropping a special 151st Power Star, a special ending, the key to an anomalous area such as a DS equivalent of the 4th floor, or a key to unlock Waluigi. Other reports say they don't drop anything at all, softlocking the player upon defeat or warping them back to the Castle Grounds. This is likely due to the A.I. not being able to generate the intended reward.