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.
Link first comes across the chamber on his mission to find the Mirror of Twilight, which he needs to access the Twilight Realm. As soon as he and Midna arrive here, a group of Shadow Beasts attempt to ambush him. After slaying the dark creatures, the young hero equips his newly acquired Spinner and rides the tall statute situated within the center of the chamber. Link can then use his Spinner again to activate a switch thus sinking the statute into the ground and raising the Mirror of Twilight. 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 Sages explain how they were tasked with guarding the Mirror and how it was shattered before they came here. One of the Sages then goes on to speak of how they were sentenced to kill Ganondorf, an evil demon who attempted to access the Sacred Realm. However, after impaling him with the Sword of the Six Sages, by some "divine prank", Ganondorf was able to acquire the Triforce of Power and break free of his imprisonment. With no choice left, the Sages sealed him within the Twilight Realm. 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.
After Link and Midna have collected all three Mirror Shards, thus restoring the Mirror of Twilight, they intend to go and confront Zant in the Twilight Realm. Before this however, the Sages appear before them. The Ancient beings apologise for their hasty actions and ask for forgiveness. They also refer to Midna as the Twilight Princess, and she goes on to explain why she chose Link and how she now understands that she must save Hyrule as well as her own homeland.
Link and Midna come here one final time during the final credits. After defeating Ganondorf and saving Hyrule, Midna prepares to say her goodbyes to both Princess Zelda and Link. However, the leader of the Twili ends up shattering the mirror thus breaking the only known link between the two worlds.