David Haller, the mutant known as Legion, originally only had 3 alternate personalities as a result of his mutation. After Bishop plunged David's own psi-blade into his chest to prevent the onset of the Age of Apocalypse, it was thought that Legion had died, and his personalities had died with him. It was later revealed that Legion had not actually died, but had in fact been banished from existence to a realm beyond time. It was there that his personalities accumulated and grew into the thousands, and therefore it is likely that Styx was born in that realm. He is personality #666 in Legion's personality matrix.
Following the disastrous events of "Age of X", Dr. Nemesis and Reed Richards work together to create a device aimed at keeping Legion's thousands of personalities under control. While they are testing out the device, they discover that some of the personalities and their powers cannot be accessed. Legion uses an omniscient personality called the Delphic and discovers that six of his personalities, led by Styx, have escaped from his mind and entered the real world after assuming corporeal forms.
A team consisting of Legion, Magneto, Rogue, Frenzy, Gambit and Professor Xavier is assembled to hunt down the rampaging personas so that David can reabsorb them into his psyche. A series of events results in Styx kidnapping Xavier and leaving a trail for David to follow into the catacombs of Rome. Once inside, Styx reveals his goal: he wants to touch Legion and absorb his soul, so that he can use the reality warping powers of Moira (the personality who created the Age of X) in order to mold the universe to his image. However, he is tricked by David and Rogue and reabsorbed by Legion.
Powers and Abilities
Styx's primary ability as one of Legion's many personas was to absorb a person's soul and consciousness with a touch, turning them into mindless puppets that he could bend to his will. Once he had contained their soul within himself, he could access their memories and abilities in much the same way as Rogue. He was able to create illusions after absorbing the soul of Charles Xavier. Styx also had abilities connected with his namesake, the river of death: he could command and control armies of flies, kill a person with a touch, and cause plants and animals to wilt and slowly die by his mere presence.
Styx is named after the river that flows through the underworld in Greek mythology.