In Astral Chain, the Legion are used by the Neuron task force in order to fight the invading chimera they were made from. Using the bond between task force and Legion the two are able to work in unison and help clean up the city together. Although there are five total usable Legion in the final product, there were quite a few more planned in earlier development. It ranged from ghosts to slime in terms of alternate Legion forms before they finally settled on the core five. While most of these were finally scrapped as Legion, some did return in the form of enemy chimera so they still live on in some way.
Astral Chain launches September 30 for Switch. Check out more information on the Legion in the developer update here!