For the event, see The Great Weapon (Event).
The Great Weapon is a tome found in Act V of Diablo III. Written by Itherael, it is dropped in Battlefields of Eternity when The Great Weapon event is completed. Obtaining the journal will add to the Field Research achievement.
It reads as follows:
The Great Weapon was created in response to a siege on the Pandemonium Fortress by Kurekas, a demon lord who rode a monstrous siege beast. At great cost to ourselves, we channeled much energy into the Great Weapon, and succeeded in bringing down Kurekas and his mount. Upon his death, the demon was drawn inexplicably into the weapon, and it has been attacking us ever since.