Role in the series
Stygian is revealed to have lived during the Equestrian Great Fear Wars when Pitch Black was still reigning supreme. After Pitch's rule left his family to vanish, Stygian was left on his own, leaving Starswirl as the only family he had left, and by the time he got greedy for power as the result of his loss, and with Starswirl casting him out and thus making him feel abandoned once more, Stygian got embittered and turned to the dark ways of Pitch himself for answers. Thus, he turned him into a Taraxippus, the boogeyman of Equestria, and later on the new Pony of Shadows, as this allowed him to cast Starswirl and his friends in the Pillars into limbo, halting their aging in the process, and allowing Stygian himself to fabricate lies of their disappearance to discourage the hope they had, and after hearing that, after Pitch killed the Tree of Harmony of ancient past to keep Celestia and her family from defeating him with the Elements of Harmony (As he knew that a few side-effects would've gone with the disappearance of the tree such as Discord's plunder weeds and didn't need to replace the Elements themselves), Starswirl planted another to act as a final stroke of hope. Thus, after Stygian was cured of his corruptive influence, the Taraxippus race swore revenge by not just going after others, but Stygian's long-lost sister Halo Epiphany, who was trapped in limbo like Starswirl and stopped the previous Pony of Shadows, hosted by a current missing immortality-cursed and eternally sad Doldrumsta Blue, from taking Celestia and Luna with the same unknown fears Pitch used against them, by sacrificing her soul to save them and earning her a fancy mark (A cutie mark that covers the entire body as a result of the highest display of talent), and joining the Guardians of Harmony, while not knowing that his brother Stygian is still alive, and as she came back from limbo eons after her imprisonment as limbo is out of sync with the UUniverses' timescale and can cause temporal distortions of many kinds without the proper spell, and after hearing right away after the events of Shadow Play that his brother was still alive, she swore to find him, and yet he must still face foes of modern days and his past, including not just his feeling of loneliness, abandonment, betrayal, and still-remnant mistrust of Starswirl that are the result of his time as the new Pony of Shadows, but also manipulative foes like Bishop Dipper, his corrupter Pitch Black, and so on.