Part 129: NIER Backstory: The LegionNIER Backstory: Part 7 - The Legion
There isn't really all that much information on the Legion available. They were apparently massive, berserking white creatures made out of salt and...that's about all there is on 'em physically. Their leader is a sentient Legion called "Red Eye" named, very originally, for its glowing red eyes... Hmm...glowing red eyes...?
La la la la! The Watchers, they dance! Red Eye is basically the avatar for the Queen Grotesquerie. It has sentience and can direct the Legion like a hive mind. There isn't a lot of info about the Red Eye either, other than it is incredibly hard to kill (see: nukes are a minor inconvenience) The identity of the Red Eye is never really touched on either.
Personally, I always liked to believe it is a really pissed off zombie Caim who's put everyone on his shit list for blowing him and his dragon pal up. The DLC for NIER mentions "a soldier of salt brings forth the Legion" and that's in the middle of about fifty other Drakengard references. So, who knows! It would be neat, but there's nothing said one way or another. All the NIER backstory junk is basically a great big rough design doc for a prequel that is never, ever going to be made.
The timeline mentions the WCS and the Legion was caused by humans coming into contact with particles from the Queen Beast when she disintegrated from the rhythm battle against Caim and Red. Apparently, coming into contact with the crap is not like a normal infection since...well...you're basically taking cells from a pan-dimensional god into your body. Evidently, while the Mama Watcher's physical form was destroyed, her consciousness still exists in some form. And she's really, really pissed about what happened to her, so this world is on her shit list. So if a human comes into contact with "maso", they basically get a forced pseudo-pact thrust onto them which is:
"Wanna join the Queen's team and destroy the planet? Nice! You get to live! Now you're now a killer giant. Get to it!"
"Refuse? Fuck you, human! You're a goddamn pile of SALT now! Eat it!"
There's no real cure to a pissed off dead god taking its revenge on the world it died in, so humanity is basically boned at this point... Especially, now that you've got angry Old One dust spread all over the four winds. So basically, humanity needed to find all that crap, vacuum it up, and toss that shit over the fence back to Drakengard's world. That may take a while...