Chamber of Secrets (Earth-2165)

"Reliable historical sources tell us this much. But these honest facts have been obscured by the fanciful legend of the Chamber of Secrets. The story goes that He-Who-Must-Not-be-Named had built a hidden chamber in the castle, of which the other founders knew nothing. You-Know-Who, according to the legend, sealed the Chamber of Secrets so that none would be able to open it until his own true heir arrived at the school. The heir alone would be able to unseal the Chamber of Secrets, unleash the horror within, and use it to purge the school of all who were unworthy]] to study the shinobi path.|- The legend of the Chamber of the Secrets|undefined"

The Chamber of Secrets was a secret dungeon located underneath the Seireitei Shinobi Academy of Bending hundreds of years in the past by Madara Uchiha, who disagreed with the other founders of the academy about the concept of blood purity. The chamber housed the ancient beast known as Garaga, who, according to legend, was to be used to purge the school of nonbender-born students. The chamber was surrounded by pillars with the Statue of Garaga at the far end, where the creature itself resided, symbolizing Madara's ability to speak Parseltongue.

History
Madara Uchiha, one of the founders of the Seireitei Shinobi School of Bending, believed that the institution should only admit pure-bloods as students, much like the Gakuzumi Institute of Kirigakure. His views, however, were widely unpopular, as most shinobi of his time accepted nonbender-borns to the point some even considered them especially gifted. The other three founders of the school favored the institution to admit all shinobi and kunoichi as their students.

Disagreeing with his fellow sensei, Shihōin left the school, but not before creating the Chamber of Secrets. The chamber was home to a monster - Garaga - a giant snake that was allegedly meant to purge the school from all nonbender-borns. Only someone able to speak Parseltongue would be able to find and open the Chamber of Secrets. Centuries after Madara's death, multiple sensei searched through the school to find the chamber, but were unsucessful, and so it was dismissed as an urban legend to scare students. As the chamber was opened in 176 AG and 220 AG, however, it was proven real.