The Great Weapon is the event that occurs in Battlefields of Eternity in Act V of Diablo III.
In their wanders, the Nephalem may find a colossal round fissure amidst the Battlefields, with an opening to the south. Within that rift, one can see a gigantic angelic siege weapon (similar to those broken and scattered around). However, instead of the golden light, it has a dark red glowing at its core.
On the south side of the sundering, two angelic scouts will welcome the Nephalem and ask to assist in the purification ritual. They theorize that purifying the Great Weapon from the evil that corrupts it will allow the Heavenly Host to use this weapon again, this time against Malthael.
Starting the ritual will cause hordes of ghostly demons to spawn: Subjugators, Oppressors and Warscarred Marauders. Players must kill them all: the progress bar fills based on the amount of demons killed, not time elapsed. Every 20 seconds, the weapon will release a darkfire blast which inflicts damage equal to half of a creature's Life, regardless of their protections, to everything on the platform. The only way to avoid this damage is to stand behind the scouts (the angels will yell for the Nephalem to come closer to them so that they can protect them). One can visually see the weapon charging up (glowing brighter), as well as angels shifting from channeling beams to holding shields.
When the bar is filled, Kurekas spawns, but no longer receives reinforcements (demons already present will remain). The weapon will still fire on the Nephalem, but it will not damage the demon lord. Killing Kurekas ends the event.
In the end, the angels will regrettably say that the Great Weapon is useless now, as Kurekas corrupted it, but at least it will not do any more harm. The weapon itself will return to its original color and stand idle amidst the fissure. Upon completion of the event, The Great Weapon lore book will also be added to the player's journal.
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Diablo 3 - Reaper Of Souls - Act 5 - The Great Weapon Event