Think about it!
The Vex=Death: The Vex are an inescapable force, immortal through time and a danger to all forms of life. They bring end to the living as if they were a key component of the nature of the universe itself.
The Fallen=Famine: The Fallen are a scavenging race, picking at the bones of the golden age. They are completely dependent on the machine gods that sustain them. All they do is consume.
The Cabal=War: The Cabal are a race of warriors, their very fundamentals based on the dominance of other intelligent species. A WARpath has been left in their wake that nothing will stop.
The Hive=Pestilence: The Hive exist as an aversion to natural order. They are a sickly force of mortality bent on a virus-like mechanism of sacrifice and mass production used to invade and hollow out worlds.
See! In all honesty, I feel like the Vex and the Hive could switch roles, but I only say that the hive represent pestilence because of their diseased appearance. Also, the enemy races heralded the dark age, did they not? Thoughts?
Edit: Regarding the conquest/pestilence debate, I used the pestilence interpretation because that's what I'm familiar with. In line with conquest:
Vex=Conquest
Cabal=War
Fallen=Famine
Hive=Death
English
#Destiny
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If that's true then the traveler would symbolize the antichrist