Portal Stone

The is the grand, black monolith housed in the Mirror Chamber in  that is not only the portal to the Palace of Twilight from the Light World, but also the very stone Ganondorf was chained to during his trial by the Sages before the events of Twilight Princess.

History
The Portal Stone is revealed from the sand in response to the lowering of the Sand Goddess statue that stood in the chamber upon Link's arrival, after defeating the dungeon's boss, Stallord. At the same time, the Mirror of Twilight rises from the sand opposite the Portal Stone. In the absence of the Mirror of Twilight, the Portal Stone is dark and stolid, however, when the Mirror is fully reformed towards the end of Twilight Princess (and as seen in the cutscene describing Ganondorf's trial) the Stone shines with Twilight markings. As more light from the Mirror hits the Stone, its rocky face contracts at its center in increasingly deeper layers, and thus provides any in the light of the Mirror passage to the Twilight Realm. These layers are notable for their inclusion of Ancient Hylian text and the Triforce symbol, the latter of which exists at the stone's deepest layer.

During Ganondorf's trial, the Stone was activated in order to imprison Ganondorf in the Twilight Realm, after he disposed of the Water Sage, in the respective cutscene in Twilight Princess. Likewise, Link must reform the Mirror of Twilight towards the climax of the title to reactivate the Stone, and enter the Twilight Realm to face off with Zant.

In the finale of Twilight Princess, Midna shatters the Mirror altogether and exits the Light World via the Portal Stone. With the Mirror shattered, the Stone returns to its dull, sheer state.

Trivia

 * When the Portal Stone is activated, the resulting opening bears some resemblance to the "inside" of a Gate of Time, as both are receding tunnels composed of layered circles (though the latter is made up of gears, while the former is seemingly composed of light). The nature of the two is obviously very different, however.
 * It is not clear if the Portal Stone itself is necessary for activating the Mirror of Twilight, or if any sheer surface will do. It is possible that the stone was simply used for the purpose of expediting the Mirror's use and ensuring it could be activated with minimal effort.