Legions are one possible result of White Chlorination Syndrome. When a human contracts WCS, it is suggested that they are given a choice between death, or binding their soul in a pact with The Watchers and thus becoming a Legion.
A human who becomes a Legion undergoes radical changes. The most obvious is the transformation into a "white monster" which is nevertheless said to look unmistakably human. Humans who die from WCS are said to end up composed entirely of salt (NaCl); it's not clear whether the same is true of those who become Legions, however. They also become violently aggressive towards other humans, much in the same way as relapsed Gestalts. Finally, particles of Legions (possibly Maso) can spread the White Chlorination Syndrome to other humans. It's not known if a way of reverting Legions to human was ever discovered.
The Legions were led by a monster called Red Eye. After its defeat in Jerusalem in 2030 by the 13th Crusade, the Legions became much less organized, and thus less of a threat to humanity. However, due to the remaining presence of infectious maso, Project Gestalt continued, leaving Replicants and androids behind to rid the world of the maso and the remaining Legions. In 3276, a Replicant traveled to Jerusalem and unwittingly caused the rebirth of Red Eye from its atomized remains. With this, the Legions once again became a major threat. After an 11 year battle, Red Eye was once again defeated and all traces of infectious maso were confirmed purged from the world, thus ending the threat of Legions and White Chlorination Syndrome.