As Firim states the 6 are 1 entity while denying yet another.
The 6 are Death created to become the nemesis of the 1 who represents the Light. If the 6-headed God of Death dies then the remaining entity becomes Death to fill the Void.
If he dies then his crown is taken and his rival becomes the 6-headed God of Death to fill the Void.
If the Crown is destroyed then the Cosmic Realm of Light itself becomes Death to fill the Void.