The Moment was a sentient weapon, which the Doctor aimed to use to end the Last Great Time War. (TV: The End of Time, TV: The Day of the Doctor)
The Tenth Doctor claimed that the Eighth Doctor created the Moment from a modified Key of Rassilon and De-mat Gun during the War. (COMIC: The Forgotten, Don't Step on the Grass) In actuality, the War Doctor stole the weapon from the Time Lords. (TV: The Day of the Doctor)
The Doctor later used it to end the war, time-locking it in the process. (COMIC: Don't Step on the Grass) When the Doctor used the Moment, he destroyed Gallifrey, the Daleks, and the Time Lords. (COMIC: Sky Jacks) The Ninth Doctor later stated that all the combatants of the Time War had burned and been "wiped out in one second". (TV: Dalek)