Weapon (The Eyeless)

According to one account, an unnamed superweapon was created by the Time Lords and used by the Doctor to end the Last Great Time War. After the war, it sat inside the Fortress on Arcopolis.

Abilities
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.

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 Tenth Doctor said that its creators 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)

During the War in Heaven, some accounts suggested that the "Greater Key" resembled a metal rod. (PROSE: A Farewell to Arms)

In the year 291994, the weapon, contained inside the Fortress, appeared in Arcopolis in the middle of the Seventh Galaxy. It was fired exactly once, by the Doctor against the Daleks, but it malfunctioned, destroying the Time Lords and most of the Daleks but leaving the Doctor and some Daleks still alive; the Doctor speculated this was due to a protective field around the weapon. This was called the Last Battle of the Seventh Galaxy. The Fortress remained in the centre of Arcopolis, the weapon still inside it.

Fifteen years later, the Tenth Doctor travelled to Arcopolis to destroy the weapon, wanting to stop it from ever being used again and knowing he was the only person left in the universe who could disable it. 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 TARDIS but became interested in the weapon. The Doctor ultimately found the weapon and activated it, destroying the Eyeless. This led the Doctor to realise that the weapon had been malfunctioning and that the ghosts were complex blast shadows left behind when the weapon annihilated them during the Last Battle.

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)

According to some accounts, this weapon that the Doctor used to end the Time War was called "the Moment". (TV: The End of Time, COMIC: Don't Step on the Grass) Some accounts held that the Eighth Doctor had built the Moment as a modified De-mat Gun that could remove its target from time, (COMIC: The Forgotten, Don't Step on the Grass) though others stated that the Moment had other powers and didn't in fact destroy the Time Lords, despite the War Doctor's initial intent. (TV: The Day of the Doctor)

Behind the scenes

 * Creator Lance Parkin has pointed out that the weapon could be safely fired, without causing its user's destruction, if it was aimed at someone outside the firer's noosphere. He says in AHistory that the War in Heaven provides an obvious candidate for an Enemy for which that would be the case, suggesting that the weapon was built for that conflict before being used in the Time War. The Faction Paradox series A Farewell to Arms involves a superweapon called the Greater Key, whose description is consistent with both the Weapon and the Moment, with a backstory matching these suggestions.