Stygian

Stygian is a unicorn stallion who appears as the main antagonist of Shadow Play - Part 1 and Part 2, alternatively known as the Pony of Shadows, an entity first mentioned in Castle Mane-ia and pictured in. He is a former strategist and ally of the six Pillars of Old Equestria, temporarily transformed into a monstrous shadow pony until he is restored to his original self by the Elements of Harmony.

Development and design
The name "Stygian" means "dark and gloomy", and is also associated with the River Styx from Greek mythology, where souls of the dead are ferried to the underworld.

Depiction in the series
The "Pony of Shadows" is first mentioned in a ghost story told to Rainbow Dash by Applejack in Castle Mane-ia. The entity is described as a ghostly apparition produced from Nightmare Moon's lingering dark magic after her banishment, and is rumored to haunt the Castle of the Two Sisters at night. The story is seemingly debunked when Twilight Sparkle and her friends unmask a hooded "shadow pony" as Pinkie Pie playing the organ while exploring the castle.

In Shadow Play - Part 1 and Part 2, the Pony of Shadows is revealed to be a real pony named Stygian. As told in Star Swirl the Bearded's secret journal, and later detailed by the Pillars of Old Equestria, Stygian is an ordinary unicorn who assembles the Pillars to protect Equestria from the sirens a thousand years ago. Despite his success in forming battle strategies for the Pillars, Stygian becomes envious of the heroes' fame and power, leading him to take the magical artifacts that give them strength. Shunned by the Pillars for this evident betrayal, a resentful Stygian transforms himself through dark magic into a monstrous, Alicorn-like shadow pony bent on revenge. Unable to defeat him, the Pillars imprison the Pony of Shadows in limbo along with themselves, planting seeds in their place that eventually grow into the Tree of Harmony.

In the present, the Mane Six inadvertently release the Pony of Shadows upon freeing the Pillars from limbo. Weakened from his imprisonment, the Pony of Shadows flees to spread darkness across Equestria and regain his former strength. The Mane Six and the Pillars retrieve the Elements of Harmony to create another portal to limbo to contain him, planning to sacrifice them in the Pillars' stead.

The heroes face the Pony of Shadows at the Well of Shade, the temple where Stygian first discovered the dark force that transformed him. Before the ponies can finish banishing him, Twilight and Starlight Glimmer see Stygian's true form inside the creature's body and attempt to reason with him. Stygian, tenuously freed from the darkness's influence, reveals that his intent was never to steal the Pillars' artifacts, but rather to make his own copies and help them defend Equestria as their equal. Realizing their tremendous error, the Pillars help the Mane Six use the Elements to separate Stygian from his alter ego, which is pulled back into limbo. Stygian is then vindicated by the Pillars and remains in the present era alongside them.

Depiction in comics
Stygian is alluded to in From the Shadows by his descendant Shadow Lock, who begins erasing all evidence of his ancestor's existence from every book in Equestria, hoping to prevent his return. Stygian's "Shadow Pony" form, unnamed, is pictured on the final page of.

Quotes
"You summon me at your peril, Star Swirl! Once I defeat all of you, this realm will embrace the darkness as I did so long ago!"

- Shadow Play - Part 1

"Don't fret. When I extinguish the light and hope of this miserable world, you won't remember any of this."

- Shadow Play - Part 2

"The darkness welcomed me when no pony would, and I will do what I must to protect it!"

- Shadow Play - Part 2

"I... I want to believe you... But the darkness will not be stopped!"

- Shadow Play - Part 2