The Moment

The Moment was the most powerful and most dangerous weapon in all of creation, described as the Galaxy Eater, which the Doctor intended to use to end the Last Great Time War. The Moment was capable of destroying Gallifrey, Daleks and whole galaxies within in a single moment, hence its given name. Additionally, the Moment was able to breach time locks and create tears in the fabric of creation called time fissures that would allow people and objects to pass from one time period to another, possessing a trans-dimensional awareness of the past and future. This extended to events that were time-locked. It also had a sentient interface with telepathic abilities, enabling it to read the thoughts and memories of those who intended to use it. The interface could also take the forms of others, such as Rose Tyler, as a holographic projection. The Moment could also change appearance from a box full of gears, to a box looking station with a big blood red ruby button on a stalk, with a base similar to the shape of a rose petal. (TV: The Day of the Doctor)

According to the General, the Moment was the final creation of the Ancients of Gallifrey. It had an operating system so sophisticated, so advanced that it had become sentient and developed a conscience (hence why it was never used, as the Time Lords feared its moral judgement). When attempting to initialise it, the War Doctor expressed unfamiliarity with the Moment's controls and operating system. (TV: The Day of the Doctor) According to another account, however, the Eighth Doctor created the Moment from a modified Key of Rassilon and De-mat Gun during the Time War. (COMIC: The Forgotten, Don't Step on the Grass)

Until the Doctor stole it, the Moment had been locked away along with other forbidden weapons — collectively known as the Omega Arsenal — in the Time Vaults. While all the other weapons were used against the Daleks during the course of the war, the Time Lords refused to use the Moment for fear that it would punish anyone who did so. When conversing with the War Doctor, the Moment warned him there would be consequences for him if he followed through with his plan. The War Doctor replied he didn't care as he had no intention of living past the war's end. The Moment responded by telling the Doctor his punishment would be to live, burdened with the knowledge and guilt of his actions. (TV: The End of Time, TV: The Day of the Doctor)

By using the Moment, the Doctor time-locked the war, making it theoretically impossible to reach via time travel. (COMIC: Don't Step on the Grass) Supposedly, when the Doctor used the Moment, he destroyed Gallifrey, the Daleks, and the Time Lords. (COMIC: Sky Jacks) As the Ninth Doctor later stated, all the combatants of the Time War had burned and were "wiped out in one second". (TV: Dalek)

However, the Doctor later learned that his memory was not entirely accurate. When the War Doctor had attempted to activate the Moment, its interface had appeared to the him in the form of a face from his future. It gave him an opportunity to change the outcome of the Time War, by involving two of his future selves, the Tenth Doctor and the Eleventh Doctor, allowing him to see the future and later providing an opportunity for the future Doctors to join him as he prepared to trigger the Moment. When the advice of Clara Oswald prompted the Eleventh Doctor to remember his vow as the Doctor, he deactivated the Moment, informing his past selves that the three of them could find another way. Together, with help from their previous incarnations and the Twelfth Doctor, the Doctors were able to devise a way to freeze Gallifrey in time, while allowing the Daleks to destroy themselves in the crossfire. Thus, Gallifrey and the Daleks still vanished, and history was preserved, but the Doctor was ultimately able to prevent the genocide of his own people. The War Doctor and his future incarnations were unable to retain the correct memories until the timeline caught up with the Eleventh Doctor. (TV: The Day of the Doctor)

Behind the scenes
The conscience of the Moment to which the War Doctor spoke was played by Billie Piper; the face from the Doctor's future was that of Rose Tyler as the Bad Wolf.

The events of The Forgotten seemingly contradict the origins of the Moment as presented in Day of the Doctor. In the comic, the Eighth Doctor was said to have built it. The television story states the Moment was created by the ancients of Gallifrey, and the War Doctor doesn't recognize it.

O Momento