The Mirror Chamber is a location in Twilight Princess, accessed after Stallord is defeated; this area is positioned on the roof of the Arbiter's Grounds. This area was where the Ancient Sages banished the worst of criminals to the Twilight Realm through the use of the Mirror of Twilight and a large unique rock they projected the portal onto.
As soon as Link arrives here, a group of Shadow Beasts attempt to ambush him, however he kills them and after riding up to the top of the Statue positioned within the chamber using his Spinner, the young hero pulled the Mirror of Twilight out of the sand it was buried in. Seeing that it was shattered, Midna gasps out in shock and anger, as she knew Zant was behind it. At this point the Ancient Sages appear before them and speak of what had happened. The Sage with the Shadow Medallion on its garb states that guarding the Mirror was something they have been doing since ancient times. The Sage with the Forest Medallion on its garb then follows up by saying that while Link and Midna were seeking it, the Mirror had now been shattered by mighty magic. The sage who wore the Spirit Medallion adds to this by mentioning how it was broken by a dark magic only he possessed, and the Sage who wore the Light Medallion then continued saying the man they spoke of was Ganondorf. This Sage then told about how Ganondorf was the demon thief who stormed Hyrule in an attempt to invade the Sacred Realm, he wielded evil magic and was renowned for his ruthlessness, somewhere down the line the Sages attempted to execute Ganondorf for his terrible crimes by stabbing him in the chest with their Execution Sword. However he survived because due to what they called a "divine prank" he had received the Triforce of Power, after this he easily broke out of his shackles and killed the Sage of Water, laughing and pulling the sword out of his chest afterwards. In response to this, the remaining Sages took drastic measures and banished Ganondorf to the Twilight Realm, mourning over the loss of the Sage he had killed shortly afterwards. Once this was all explained the sages brought up the notion that perhaps Zant had gotten his powers from Ganondorf, Midna commented saying that they figured that out too late, and explained how only the true leader of Twilight could shatter the Mirror completely while Zant was only able to break it into a few pieces. The sages then inform the two of them where these fragments are located, one in the snowy mountain heights, one in a ancient grove, and a final one in the heavens. The sages them stated Link would likely be able to collect the shards, but warned him of the impending dangers that were ahead, knowing what must be done now Link and Midna left the Mirror Chamber in search of the Mirror Shards.
Later once Link and Midna return after reassembling the Mirror of Twilight, the Ancient Sages appear before them once again, this time they apologize to Midna for allowing Ganondorf into the Twilight Realm and soon it is revealed that Midna is the Twilight Princess. Midna is surprised to hear that they knew this but soon explains about what had happened to her, stating that Zant has placed a curse on her and that because she fled she could never forgive herself and be the Ruler of Twilight again. Midna continues saying that the reason she chose to help Link out was because in her realm it was said a Hero would appear in the form a divine beast, and Link's Wolf form was very convincing, at first she only cared about her own needs but this fact changed once she saw the sacrifices both Link and Princess Zelda made. Now understanding that both the world of light and shadow had to be returned to normal, Midna motivated Link to continue onward for the sake of everyone, and after this the Sages disappear and the two of them proceeded to enter the Twilight Realm.