Greater Key

The "Greater Key" was a code name given by Huxley to a Great Houses superweapon of immense power. The Key was sentient, and had the ability to rewrite history in major ways.

Nature
Huxley's sources disagreed on what the Greater Key looked like. Most said it was a box with circular writing. (PROSE: A Farewell to Arms) While in the custody of P.R.O.B.E., it looked like "brass-studded music-box" with a red button on top. However, the Song of Aten's Arsenal, the Gendar legend about the Key that Maxie Masters had heard, apparently described it "differently"; (PROSE: Out of the Box) However, one person was "certain" that it was a metal rod, which was the forms it took when Wade first found it. (PROSE: A Farewell to Arms)

The Greater Key was sentient, comparable to "an imprisoned djinn" from Earth folklore: a "transdimensional consciousness" was imprisoned within the device. While sitting next to the Greater Key on the Dr Smith shelf, Box heard it "humming to itself" about eating galaxies, and elected not to try to communicate with the other intelligence.

Giles was warned, when he was given custody of it, that the Greater Key was a weapon of last resort, because it could unpredictably harm the user as well as the foe it was being used against. Maxie explained that according to legend, when activated, the Greater Key would rewrite history to "a version of events where [one] didn’t, actually, have to press the button". However, again like a bottled djinn, the Key did not necessarily achieve this result in the way one might wish. It was liable to rewrite the user's life to mold them into someone who would never dare to activate the weapon, instead of doing anything about the actual threat. (PROSE: Out of the Box)

Theft from RMS
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, 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)

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) The Gendar, who worshipped the Sun Builders as their gods, had a legend about the Greater Key entitled the "Song of Aten's Arsenal", with which Maxie Masters was familiar. It used another name for the device than "Greater Key", but Maxie instantly recognised the Key from Giles's description. (PROSE: Out of the Box)

Notably, on the last day of the Last Great Time War, the War Doctor removed a superweapon that could be described as "a box with circular writing" from the Omega Arsenal; it was known as the Moment or "the Galaxy-Eater". 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)

On Earth
Well after it had passed into the legends of the Gendar, an unknown party dropped the Greater Key on Earth, in the custody of the Preternatural Research Bureau, in the form of a "brass-studded music box". They left a note, explaining that the offering was "a token of appreciation from their master" and that the weapon was only to be used in the greatest of emergencies. Director Giles placed the Key on the Dr Smith shelf, along with many other artefacts donated to the Bureau by time-actives. Box, an alien supercomputer, was placed not far from the Greater Key on the shelf; she attempted to communicate with the Key, but heard it humming to itself about "eating galaxies" and elected not to attempt further contact.

Years later, when explaining to Maxie Masters why the objects on the Dr Smith shelf were dangerous and not to be touched, Giles took the Greater Key as an example. Maxie, a Gendar refugee recently hired by P.R.O.B.E., recognised the "Key" as the subject of the Song of Aten's Arsenal and told Giles more about how it was said to function, and the dangers of using it lightly. She had not previously believed the Key to be real, and was shaken by the revelation that it was on Earth, right next to the more innocuous Box. When she was told about the note speaking describing it as a token of appreciation from the "master" of whoever had left it with P.R.O.B.E., she speculated that the person who gave the Greater Key to P.R.O.B.E. may have been a Gendar priest, although Giles was unconvinced. (PROSE: Out of the Box)

Behind the scenes
The Greater Key is heavily implied to be the Moment. The code name "Greater Key" matches 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, and further asserts that the Time Lords built the Moment for the War in Heaven but only used it during the Last Great Time War, matching the history of the Greater Key.

In light of these implications, the legend of the Greater Key described in Out of the Box appears to function in part as a proposed commentary on the in The Day of the Doctor of the existence of the War Doctor, and the fact that the Doctor had not actually destroyed Gallifrey. The implication is that, after the Eighth Doctor decided to use the weapon, it rewrote history to replace his role in the Time War with the War Doctor, a version of him who ultimately elected not to press the button.