D) Moon Spark

The original Pony of Shadows

"You hold the answer deep within your own mind. Consciously, you've forgotten it. That's the way the human mind works. Whenever something is too unpleasant, too shameful for us to entertain we reject it.
"But the imprint is always there. Nothing is ever really forgotten."
-- Evanescence, Understanding

Ongoing antagonist

Female

Two thousand years old, give or take

Moony (Shadow Spark?)

Everything Ends Here - BlueLionMusic
Master of Shadows - Symphony of Specters
Far From Home - Sam Tinnesz
Wolves - Sam Tinnesz
Understanding - Evanescence
Shattered - Myuu

Average unicorn height
Height as Shadow Spark increases to about Celestia's height, maybe just below

Electric yellow

Cutie mark is an exploding moon (that's the only way I can think of to describe it X)

Average

Blueish indigo

Unicorn

Green

Short, spiky, unruly

Average

Lightning manipulation
Dark/Shadow Magic

PTSD
ASPD
Amnesia

Manipulative
Persuasive

Finding her memories

Lack of foresight
Lack of empathy
Doesn't see consequences

Talented ponies think they're superior

(As a shadow) Lurking around the Castle of the Two Sisters

ISTP

None

Lightning
Shadow spears [Ascension weapon]

A little pebble *

Red

EVIL SORCERESS, MWUAHAHAHAHAHAHAHAHAH

N/A

None

Unknown

Moon Spark was the sister of Star Swirl, and was born just before the Migration to Equestria. They had to travel through the Crystal Mountains, where a fierce snowstorm separated Moon Spark's group, including her father, from the rest of the Migration. They were trapped there for some time before The Nightmare Forces found them and chose young Moon Spark as a host. She entered Magic Overload and blew a hole straight through the mountains, creating The Broken Pass and entering a deep sleep.
She awoke years later and wandered to central Equestria.

As she was born just before the Migration, she did not receive any sort of training in magic.

Pet rock. X)

The pebble was given to her by Stygian. She threw away the pebble one day, in anger. Or maybe lost it.
Over a thousand years later, a certain deadpan Earth Pony finds it, and names it Boulder