Nyx

Nyx is a black alicorn filly who first appeared in a minor role in Pen Stroke's fan-fiction work Creeping Darkness and was later a major character in Past Sins (written by the same author). In the first story, she was Luna cursed to take the form of Nightmare Moon as a foal. In the second, she was Nightmare Moon resurrected as a filly.

Nyx in Past Sins
Nyx is best-known for her central role in Past Sins. She was created by an unfinished ritual intended to restore Nightmare Moon's power and give her a body of her own. When Twilight Sparkle discovers her, she remembers nothing from longer than two hours earlier, but possesses an understanding of the Equestrian language and pony behavior.

Twilight takes the filly in to determine if she is actually Nightmare Moon, but soon comes to care for Nyx like a daughter. Possibly as a result of Twilight's blood being used in the ritual, Nyx has a nearly insatiable appetite for learning and loves to read as much as her adopted mother does.

She is also extremely sensitive and prone to crying fits when she thinks she has done anything to disappoint Twilight. During the story, she is called a "crybaby" numerous times. Her love of learning makes her unpopular with the colts and fillies in her class, but she manages to befriend the Cutie Mark Crusaders and Twist.

As an alicorn, Nyx possesses incredible magical ability - best demonstrated by her ability to turn Fluttershy into a tree with relative ease.

She wears a vest to hide her wings and a pair of enchanted glasses to disguise her eyes, which naturally resemble those of Nightmare Moon.

Reception
Nyx's reception within the fandom has been mixed, but notably with more nuance and positivity than that of alicorn OCs in general. Fans of Past Sins adore Nyx and find her to be cute and likable, whereas the story's detractors call her a Mary Sue and criticize the perceived derailing effect she has on Twilight's personality throughout Past Sins.

Appearances

 * Creeping Darkness, by Pen Stroke and Batty Gloom
 * Past Sins, by Pen Stroke and Batty Gloom
 * Fandom Crisis! by Mr Boltitude