the great Journey is a ritual of the Conveneant. the Prophets are thinking that if this happens, they will become gods. but actually it means the Extinction of everything... and the arbiter thinks, the Great Journey is a... well a Great Journey XD
the Conveneant:
A bound of Alien races like the elites, the Brutes, the Grunts, The High Prophets, the Hunter, the Drones, and the Jackal. they all think that Humanity is heretic race in their Religion and they want to extreminate Humanity for this.