Vincent had first heard of the Celestial Beasts that turned out to be Demihumans. They were an alien race that followed the Celestials here to take control of the land or some other reason he doesn't know.
There is also the Xonid Legion which he already encountered after it parasitized a Celestial's body.
The other three were the Arkhan Legion, Thoran Empire, and the Morian Collective.
Even when Luna was talking about them, she didn't seem well and didn't want to recall her experience with these aliens.
Needless to say, they are troublesome opponents because how most of them were capable of wielding Aether, a different type of energy that can corrupt things…
'Hmm? Corruption?' As Vincent reached this thought, he immediately had an idea.
