"Re-forged by Ulthane, the Armageddon Blade is the only weapon powerful enough to slay the Destroyer."
The Ultimate Blade -- to acquire it, the player must assemble it's seven shards that have been scattered throughout the game's world to be re-forged by Ulthane. Superior to War's Chaoseater, the Armageddon Blade is generally the ultimate tool to destroy your enemies. Unlike the Chaoseater, however, it cannot be equipped with enhancements, and once obtained it replaces the Chaoseater and can not be changed back.
You begin the search for the shards after all the chosen are defeated and Azrael is freed from The Black Throne. Using the Mask of Shadows, War finds the shards which are hidden around various areas of the ruined city.
You first see this blade in a cutscene where Ulthane has made it so that Abaddon can destroy 6 of the 7 seals in their plot to vage war against hell without the Horsemen interfering.