Pony of Shadows

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

- Pony of Shadows

Stygian, better known as the Pony of Shadows, is the main antagonist in My Little Pony: Friendship is Magic in the Season 7 finale, Shadow Play. He is an astronomer and the founder of the pillars, who was also their friend as he brought them together to save his home and Equestria. After the rescue, Stygian was outcasted by everyone, including the pillars, after he was unnoticed. He was then transformed into a monstrous Pony made entirely of shadows and had desires to cover the entirety of Equestria in darkness before he was reverted back to his original form.

He is voiced by Bill Newton, who also voices Bright Macintosh and Pharynx in the series.

Background
The Pony of Shadows was originally a male unicorn named Stygian, who lived in Equestria 1,000 years prior to the events of the series. When Stygian's home was attacked by the Sirens, he summoned the help of the Pillars of Old Equestria: A group of 6 powerful ponies who had the power to defeat the Sirens. Their leader, Starswirl the Bearded, put an end to the Siren's reign by banishing them to another dimension, causing the Pillars of Old Equestria to be hailed as heroes, while Stygian went unaccredited for his actions.

Stygian begins to feel outcasted by this, and decides to use a spell to gain a better understanding of the Pillars of Old Equestria. However, this spell requires a specific item from each of the members, and his theft of these items leads the members to believe his spell is intended to steal their power. Angered, the Pillars of Old Equestria banish Stygian. Having been betrayed by his own heroes, hatred festers inside of Stygian, transforming him into the Pony of Shadows.

The Pony of Shadows attempts to attack the Pillars of Old Equestria, but they use their combined magic to seal him in limbo. This ritual also causes themselves to become trapped in limbo as well, so they plant the seeds for what would eventually become the Tree of Harmony to watch over Equestria in their absence.

Escape
Over a thousand years later, the Pony of Shadows is inadvertently released from limbo by Twilight Sparkle, once she and her friends learn of Starswirl being trapped. The Pony of Shadows then destroys Stonehenge and Starswirl's spell book so he can't be sent back to limbo and attempts to attack Starswirl, however, Twilight and Starlight Glimmer counter his attack, forcing him to retreat to shadowy areas of Equestria to regain his power. However, due to the industrialization of Equestria over the past millennium, many of the areas that were once known to be dark are now populated and bright, thus buying Twilight and her friends time to come up with a plan on how to locate and re-contain him.

Eventually, the ponies are pointed out by the castle's map the location of The Pony of Shadows; an abandoned ghost town on the edge of Equestria. They travel there, and the entire town collapses into the ground to reveal a temple, where the Pony of Shadows resides. They begin to use their magic to open up a portal to banish him, while Starlight attempts to urge Twilight to rethink their plan, believing Stygian can still be saved.

Defeat
As the Pony of Shadows begins to slip into the portal, Twilight hears a voice from within the black mass that is his body, and dives in. She meets Stygian, sitting alone in the darkness. Stygian explains the entirety of the situation to her; in reality, Stygian actually wanted to make copies of the Pillars' artifacts so he could be heroes like them and never wanted to steal their powers. Unbeknownst to Twilight and Stygian, the Pillars and the rest of the Mane Six overheard their conversation, and the Pillars were shocked and horrified, now knowing they were wrong and realized they really should've asked and let him explain his true intentions. As the darkness begins to overtake Stygian again, Starlight joins Twilight and explains that she was like Stygian until Twilight managed to convince her of the power of friendship, although the darkness still pushes her and Twilight out. Twilight, Starlight, the Mane Six and the Pillars then combine their magic to pull Stygian out of the darkness, and the Pony of Shadows, without Stygian, cannot prevent himself from being sucked into the portal and banished back to limbo. With the Pony of Shadows gone and the Elements of Harmony and Pillars of Old Equestria both intact, the land is at peace once again, and Starswirl apologizes to Stygian and rekindles their friendship.

Appearance
Before his transformation, Stygian was a weak, skinny unicorn with a gray coat and a blue mane. He wears a brown coat over his body. His horn is tinier than any other unicorn's horn.

After becoming The Pony of Shadows, he takes on the form of an amorphous dark gray Alicorn with glowing white eyes and wispy, smoke-like wings. His horn becomes large and curved, much like King Sombra.

In both forms, he speaks in an indistinct European-sounding accent. In his mutated form, his voice also has a filter effect similar to what Queen Chrysalis had in Season 2, albeit much deeper.

Stygian was reverted back to his original form after being separated from The Pony of Shadows.

Personality
Before becoming evil, Stygian was rather soft-spoken and non-confrontational. Rockhoof refers to him as "scholarly".

As the Pony of Shadows, he becomes much more arrogant and loud. He constantly belittles the Pillars of Old Equestria and mocks Twilight for unknowingly setting him free. Even until the moment of his defeat, he maintains his composure and acts unfazed.

After being separated from the darkness and reconciling with Star Swirl, Stygian forgives him and the Pillars and returns to being a happier and kinder pony.

Trivia

 * A mythical entity referred to as "The Pony of Shadows" was mentioned in the Season 4 episode, "Castle Mane-ia". However, this event is never mentioned in the Season 7 finale, so it is unknown of these two entities are one in the same.
 * Not to mention that the aforementioned entity was explained as being the accumulated hatred of Nightmare Moon, whereas the entity featured in Season 7 is something completely different.