Greater Key

The "Greater Key" was a code name given by Huxley to a Great Houses superweapon.

Nature
Huxley's sources disagreed on what it looked like, with most saying it was a box with circular writing but one person certain it was a metal rod. (PROSE: A Farewell to Arms)

Fought over in the War in Heaven
In the War in Heaven, the "Greater Key" was kept at a Great Houses base on RMS in the Amazolian system. The Enemy attacked the base to try to obtain the superweapon, resulting in a conflict that ended after a conceptual predator got out of control and caused both sides to forget about the base and the "Greater Key".

During the Armistice
During the Armistice, Huxley sent Wade, Nezf, Tabrenilsodvoravitas, and a Shift to RMS to steal the "Greater Key" during the apparent negotiations. Wade found the "Greater Key", at the time taking the form of a metal rod. It showed her the entirety of time and space and her place in it, and was then used by Wade to destroy RMS. (PROSE: A Farewell to Arms)

Later fate
Huxley speculated that after the end of the War in Heaven the "Greater Key" would be taken back along with all the other Great Houses weapons to "some impregnable arsenal". (PROSE: A Farewell to Arms)

Notably, the War Doctor later removed a superweapon that could be described as "a box with circular writing" from the Omega Arsenal, the Moment; he attempted to use it to destroy Gallifrey and end the Last Great Time War, (TV: The Day of the Doctor) something which other accounts suggested the Eighth Doctor had achieved using a weapon described as a "metal rod". (PROSE: The Eyeless)

Behind the scenes

 * The Greater Key is heavily implied to be the Moment. By authorial intent, the code name "Greater Key" refers to the Moment's origins given in the comics The Forgotten and Don't Step on the Grass as an upgraded version of the Great Key of Rassilon.
 * In AHistory, Lance Parkin equates the weapon seen in his novel The Eyeless with the weapon mentioned in The Forgotten and the Moment as mentioned in The End of Time. He further suggests that the Time Lords built the Moment for the War in Heaven but only used it during the Last Great Time War. This matches the history of the Greater Key.