The Moment

The Moment, previously called the Eye of Discord, 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.

Abilities
The Moment was capable of destroying Gallifrey, the Daleks, and whole galaxies within a single moment, hence its given name. (TV: The Day of the Doctor) Not only could it destroy all traces of an enemy, it would destroy all traces that the enemy had existed as well as everyone who had ever even heard of them. When it was working as intended, it was capable of destroying all living things in the universe. (PROSE: The Eyeless)

Additionally, the Moment was able to 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. 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 also had the capacity to take the forms of others, such as Rose Tyler, as a holographic projection. As it was a Time Lord construct, it was not limited to a linear grasp of time and space — it could select the forms of people from an individual's future they had yet to meet. However, when trying to tell the past and future apart, the Moment tended to get the two mixed up.

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) Inside the Fortress, the weapon resembled a humming cylinder of burnished metal, one metre long and ten centimetres wide, with no controls or inscriptions. However, more interesting things were going on in higher, lower, and sideways dimensions; when she held it in her bag, Alsa could feel something beneath, inside, and around the metal, as if it was eating away at her. In this form, the weapon was activated by touch. (PROSE: The Eyeless)

History
The device was once known as the Eye of Discord and was created by Time Lord scientist, Roppen. Rassilon later employed Roppen to redesign the device to be deployed against the Nestenes. The device gave Rassilon a brief encounter with his future selves before Rassilon activated it and destroyed all the Nestenes in the Illia galaxy. He then placed the device in the Omega Arsenal for safe keeping. (PROSE: Pandoric's Box)

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)

The Tenth Doctor said that the Time Lords built it as a weapon of last resort, to be used "only when all was lost", since it would destroy not only its enemy but also its user. It ran on quantum physics, the observer effect, the superforce, vunktotechnology, and vundatechnology. (PROSE: The Eyeless)

According to one account, 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 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) The Partisan informed the High Council that the Doctor "possess[ed] the Moment, and [would] use it to destroy Time Lords and Daleks alike," while the Chancellor explained that the Visionary predicted that the Time Lords would die on the last day of the Time War. (TV: The End of Time)

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 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)

The Tenth Doctor believed that it needed to be destroyed to stop it from being used again, and he was only person left in the universe who could destroy the weapon, so he travelled to Arcopolis to deactivate it where it sat in the Fortress, where it had been left after it was used to destroy the Time Lords and Daleks alike in the Last Battle of the Seventh Galaxy. As he endeavoured into the Fortress, he was attacked by ghosts, which could pull anything they touched out of reality, and the Eyeless, who had come to scavenge the weapon. The Eyeless activated the weapon, but it backfired, destroying them and leaving the Doctor alive. This led the Doctor to realise that the weapon had been malfunctioning: the ghosts were complex blast shadows left behind when the weapon annihilated them during the Last Battle; this also explained why the weapon hadn't destroyed the universe during the Last Battle, which the Doctor had previously speculated was due to a protective field around the weapon.

The Doctor and Alsa ultimately destroyed the weapon by dropping it into a plasma vent on an asteroid. There, it opened and something came out that the Doctor and Alsa couldn't look at; the thing couldn't survive in the universe, and with it, the weapon was destroyed. (PROSE: The Eyeless)

Behind the scenes

 * In AHistory, Lance Parkin suggested that the Time Lords built the Moment for the War in Heaven but only used it during the Time War.