Omega Armageddon

Omega Armageddon is the battle that occured before the Crisis Impact, which held the fate of the universe at stake.

Ultraman Geed
Sometime after Belial regained the Giga Battle Nizer after killing Reibatos, Belial started the Omega Armageddon alongside Kei Fukuide, the Darkness Five and his new army called Terror The Belial (テラー・ザ・ベリアル) in a heated battle with the Ultra Warriors from the Land of Light at Side Space Universe, which held the fate of the universe at stake. During the battle, Ultraman Hikari created the Ultra Capsules to preserve the powers of the Ultra Warriors, hoping it could change outcome of the battle. The battle also caused the destruction of many planets in the universe, notable ones are the homeworld of the Alien Shadow race, Planet Sturm and Earth as well. After defeating Zero on Earth, Belial denotated the Super-dimensional Eradication Bomb which allows him to trigger the Crisis Impact, and the aftermath events.

Aftermath
The presence of the Ultra Warriors during the said battle had became a popular legend, with records of their existence being mysteriously lost and the memories of them slowly fading from the collective consciousness of humanity.

Trivia

 * The theory of Belial having an army during the said battle is confirmed due to the presence of Kei Fukuide, the Darkness Five and his new army called Terror The Belial. This theory was mentioned by Zero during the invasion of Alien Shadow's homeworld. Also, Zena admitted that the initial reason he joined AIB was to restore Planet Shadow's glory.