Nyx

Nyx is a black alicorn filly who first appears in a minor role in Pen Stroke's fan fiction work Creeping Darkness and is later a major character in Past Sins, Nightmare or Nyx?, Trinkets, and Winter Bells (written by the same author). In the first story, she is Luna cursed to take the form of Nightmare Moon as a foal. In the second and third, she is Nightmare Moon resurrected as a filly. Pen Stroke stated that all authors could use Nyx or any of his characters as long as the writer respects the characters, does not use overly mature themes on the characters, and gives credit to him for creating the characters.

Nyx in Past Sins
Nyx is best-known for her central role in Past Sins. She is 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. However, in Nightmare or Nyx?, her magical power is significantly weakened, due to Luna taking back the bulk of her power (it originally being hers, as Nightmare Moon) at the end of Past Sins.

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.

In JusSonic's stories, Nyx has random adventures with her adopted mother, her adopted father, Ben Mare, the adopted son of Princess Celestia, Twilight's friends, who became Nyx's honorary aunts and the Cutie Mark Crusaders. She is considered a pariah for her Nightmare Moon past. On her first birthday (coincidentally the first anniversary of Past Sins' debut on the Internet), Nyx got a pet Moon Dragon named Phobos, yet she also considered him a best friend. JusSonic made Nyx and Phobos similar to Lilo and Stitch in some ways.

Twilight Sparkle
What began as a way to see if Nyx was really like Nightmare Moon, Twilight soon grew soft on Nyx and decided to adopt her as her own. She's very protective of Nyx, like when she disguised Nyx to avoid anyone quickly judging her as Nightmare Moon. Even when in he Nightmare Moon form, Twilight still loved Nyx with all her heart and adamantly believed Nyx was still a good pony unconditionally inside. When Nyx becomes a filly once more, she has adventures with her adopted mother in the fanfics they are featured in. At times, she is both stern with Nyx, disciplining her for any bad actions and occasionally dotes on her, showing how much she means to Twilight.

Spike
Spike became weary of Nyx the moment he first saw her, thinking she was Nightmare Moon, ready to attack, but softened up to Nyx when she reveals her gentle nature. After Nyx's Nightmare Moon ruling, he feared her, but overcame this feeling and soon gave her his trust once again. In JusSonic's stories, Spike is an older brother-figure to Nyx and Nyx even loved Spike like a brother.

Applejack
Applejack became friendly to Nyx when they met each other. She still trusts Nyx even after her Nightmare Moon experience. In JusSonic's stories, she became one of Nyx's honorary aunts.

Fluttershy
Fluttershy bonded with Nyx when the 2 shared the same timid and sensitive behavior. Even after Nyx's experience as Nightmare Moon, Fluttershy is still loyal to Nyx, and managed to get Rainbow Dash to forgive Nyx when she brought up how Twilight would feel if a prison penalty was given. In JusSonic's stories, she became one of Nyx's honorary aunts, and she somehow inherited her Stare.

Rarity
Rarity was the first one of Twilight's friends Nyx met. Rarity became weary of the filly, but got to know her more to the point of trusting her and was the only one (other than Spike) to know of what Nyx looked like before Nyx became Nightmare Moon. She commends Twilight for reforming Nightmare Moon and tells her to keep the parenting up. In JusSonic's stories, she became one of Nyx's honrary aunts.

Rainbow Dash
When Rainbow Dash first met Nyx, she was unimpressed with Nyx's sensitivity and calls her a cry baby, but Nyx wins Rainbow Dash over eventually. She was upset with Nyx after what she has done as Nightmare Moon, but after an argument with Fluttershy, Rainbow Dash relinquished her animosity. In JusSonic's stories, she became one of Nyx's honorary aunts.

Pinkie Pie
Pinkie Pie was the most eager to meet Nyx, although the filly became fearful of her randomness. She decided to forgive Nyx for what she has done as Nightmare Moon simply because of a party she was gonna throw to thank Nyx for saving Ponyville from creatures. Despite Nyx's  fear of her erratic behavior, Nyx accepted Pinkie Pie and grew to love her. In JusSonic's stories, she became one of Nyx's honorary aunts.

Princess Celestia
When Princess Celestia acknowledges Nyx, she became curious when Twilight claims Nyx is her cousin. After researching Twilight's family and discovers the cult used to resurrect Nightmare Moon might have worked, she suspected that Nyx might be Nightmare Moon. After confronting Twilight about the manner, Twilight submits to an argument and hands Nyx over to Celestia. Celestia felt guilty for taking Nyx when the latter formed a mother-daughter relationship with Twilight and after battling the ressurect NIghtmare Moon, she is banished to the sun. After Nightmare Moon mellows down and releases Celestia,,the sun princess punishes her gently by letting Twilight take custody of her. In JusSonic's stories, she became Nyx's adopted grandmother when her adopted son, Ben Mare, becomes Nyx's adopted father.

Princess Luna
Luna faced off against Nyx when the latter was in Nightmare Moon form and NM eventually banished her to the moon. After being released, she went around to the mane six and others to see if Nyx should be forgiven or punished. In JusSonic's stories, she becomes Nyx's adopted grand aunt and Luna is quite fond of Nyx.

The Cutie Mark Crusaders
Nyx becomes friends with and became a member of the CMC after being defended by Apple Bloom from bullies. Even when she becomes Nightmare Moon, the CMC still trust her, even when she reluctantly locks them in a dungeon. In JusSonic's stories, Nyx and the CMC are still best friends and they often cause mischief together.

Ben Mare
Nyx first met Ben Mare when he was in a demon pony form and defended her against some timberwolves. Upon learning of the entire story of the Demon Pony's true idenity, Ben Mare, Celestia's adopted son, who was turned to a monster by a cruel unicorn named Boris and his cronies, she gradually called Ben "daddy," since Ben Mare is Twilight's love interest. When Ben Mare returns to normal, he humbly accepted the role of being Nyx's father. His relationship with Nyx is just like Twilight, he dotes on Nyx and is stern with her at times.

Shining Armor
In the Pen Stroke stories, Shining Armor fell victim to NM's mind controlled and when Nyx was returned to normal and became good, Shining Armor was very judgmental toward her when he felt that she would hurt his sister. His discrimination caused him to lose the trust of his wife and sister. After a wild adventure and some other stuff, Shining Armor accepted Nyx and eventually earned back his wife and sister's trust. In JusSonic's stories, he was more accepting of Nyx and had no qualms to treat her gently and let her be his niece.

Princess Cadance
In both Pen Stroke and Jussonic's fics, Cadance accepted Nyx as her niece and wasn't quick to discriminate against her.

Diamond Tiara and Silver Spoon
Of all the enemies Nyx has made, Diamond Tiara and Silver Spoon (mostly the former) are the worst. In Past Sins, they cruelly ridiculed her and sent her in the Everfree Forest in revenge for asking questions before recess. Their hostile relationship intensifies after Nyx's Nightmare Moon experience. In JusSonic's stories, Diamond Tiara still has her unscrupulousness, frequently calls Nyx "Nightmare Moon", and has no reservation to make her life miserable while Silver Spoon becomes more gentle and less mean toward her.

Trixie
In Pen Stroke's stories, Nightmare Moon defeated Trixie when the latter was substituting the element of magic (since Twilight was absent due to being a prisoner in Nyx's castle), and kept Trixie's hat and cape when she dropped them. She imitated Trixie with the clothing, hoping to make someone laugh, with little success, although Twilight found it funny. Despite not having interactions with Trixie in JusSonic's stories (as of yet), she stroke up a conversation with Trixie, who she calls "Ms. Lulamoon," in one fanfic, where she returns her hat and cape and manages to help her change her mean-spirited ways and befriends her. This shows that Nyx is very forgiving to anyone who antagonizes Twilight, the one pony who means a lot to her, since Trixie has done many cruel things to her.

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
Additionally, a short story sequel to Past Sins will be featured in the printed edition of the book, and remain exclusive to it for about two months.
 * Creeping Darkness, by Pen Stroke and Batty Gloom
 * Past Sins, by Pen Stroke and Batty Gloom
 * Nightmare or Nyx? by Pen Stroke
 * Trinkets by Pen Stroke
 * Winter Bells by Pen Stroke
 * Fandom Crisis! by Mr. Boltitude
 * Nightmare Night and Nyx by RealityCheck
 * Curse of the Demon Pony by JusSonic
 * Ben's First Day by JusSonic
 * Happy Birthday, Nyx by JusSonic
 * The Return of an Old Friend by JusSonic
 * Black Pony of the Family by JusSonic
 * Lorcan's Wrath by JusSonic
 * Big Brother Responsibility by JusSonic (cameo)
 * Tirek Reborn by JusSonic
 * A Dragon's Tears by JusSonic (cameo)
 * The Spirits of Hearth's Warming by JusSonic
 * Return to the Battlefield by JusSonic
 * Forgiveness Heals All Wounds by JusSonic
 * Spike's Time Off by JusSonic
 * Life with Ben by JusSonic
 * Magic Duel Aftermath by JusSonic
 * Loss of a Phoenix by JusSonic
 * Peewee Returns by JusSonic
 * House of Pony: Season 1 by JusSonic
 * Snowed in at the House of Pony by JusSonic
 * House of Pony Holiday Specials by JusSonic
 * House of Chaos by JusSonic