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 the United Pony of Goodness Universe (or 'UPGverse' for short), Nyx has random adventures with her adopted mother, her adopted father, Ben Mare (short for Benjamin Maregillian), the adopted son of Princess Celestia, Twilight's friends, who become 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' friendship similar to Lilo and Stitch in some ways.
In Reality Check's stories, Nyx emerges as the effective leader of the Cutie Mark Crusaders but never totally overshadows the three original members. However, she is quick to expand the membership to include ponies who already have cutie marks, such as Twist, and even to those outside the main three pony races such as Flitter, a changeling nymph.
Reality Check's Nyx has reconciled with Shining Armor and Cadance during a visit to the Crystal Empire (Shining Armor even going so far as teaching her to use her unicorn magic more effectively). She has also reached out to those who have previously been her adversaries like members of the Children of the Nightmare. She has been shown as being very afraid of losing her mortal friends and family (in Reality Check's writings, alicorns are eternally-lived) and was overjoyed at Twilight Sparkle's ascension to becoming an alicorn.
In MaskedPony's stories, Nyx is shown as a hero figure to Equestria, since she is the new holder of the Equestrian Emeralds. Nyx, despite being Twilight's adoptive daughter, also appears to have a adoptive father, Lunar Eclipse, the long lost son of Luna who went rogue and disappeared for 1004 years after being blessed from Dark Shadow, a space dragon who became her friend and has mysterious powers, Tex, a friend who has a strange ability, Glowing Aurora, and a sister who is created from her own blood by another cult, and protective of Nyx's safety, Dark Nyx. Nyx also shows that her relationship to her friends, Twilight's friends, and her newly godmother, Rainbow Dash has grown over the years she's been living in Ponyville. Nyx having the Equestrian Emeralds makes a strong similarity to Sonic having the Chaos Emeralds.
What begins as a way to see if Nyx was really Nightmare Moon, Twilight soon grows to care for Nyx and decides to adopt her as her own. She's very overprotective of Nyx, like when she disguised Nyx to avoid anyone quickly judging her as Nightmare Moon. Even when in her Nightmare Moon form, Twilight still loves Nyx with all her heart and adamantly believes Nyx is 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 becomes wary of Nyx the moment he first sees her, thinking she is Nightmare Moon, ready to attack, but softens up to Nyx when she reveals her gentle nature. After Nyx's Nightmare Moon ruling, he fears her, but overcomes this feeling and soon gives her his trust once again.
Applejack becomes friendly to Nyx when they meet each other. She still trusts Nyx even after her Nightmare Moon experience.
Fluttershy bonds with Nyx when the two share the same timid and sensitive behavior. Even after Nyx's experience as Nightmare Moon, Fluttershy is still loyal to Nyx, and manages to get Rainbow Dash to forgive Nyx when she brings up how Twilight would feel if a prison penalty was given.
Rarity is the first one of Twilight's friends Nyx meets, in order to create her disguise. Rarity is at first wary of the filly, but gets to know her more to the point of trusting her and is the only one (other than Spike) to know Nyx's true origins before Nyx becomes Nightmare Moon. She commends Twilight for reforming Nightmare Moon and tells her to keep the parenting up.
When Rainbow Dash first meets Nyx, she is unimpressed with Nyx's sensitivity and calls her a crybaby, but Nyx wins Rainbow Dash over eventually. She is upset with Nyx after what she does as Nightmare Moon, but after an argument with Fluttershy, Rainbow Dash relinquishes her animosity.
Pinkie Pie is the most eager to meet Nyx, although the filly becomes fearful of her randomness. She decides to forgive Nyx for what she does as Nightmare Moon simply because of a party she was gonna throw to thank Nyx for saving Ponyville from the creatures. Despite Nyx's fear of her erratic behavior, Nyx accepts Pinkie Pie and grows to love her.
When Princess Celestia meets Nyx, she becomes 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 suspects that Nyx might be Nightmare Moon. After confronting Twilight about the manner, Twilight submits to an argument and hands Nyx over to Celestia for testing. Celestia feels guilty for taking Nyx when she learns that the filly has formed a mother-daughter relationship with Twilight, and after battling the resurrected Nightmare Moon, she is banished to the sun. After Nightmare Moon releases Celestia and Luna, the sun princess asks Luna to decide what to do with Nyx, and reconciles with Twilight.
Luna faces off against Nyx when the latter was in her Nightmare Moon form, and Nightmare Moon eventually banishes her to the moon. After being released, she goes around to the mane six and others to see if Nyx should be forgiven or punished.
Nyx becomes friends with the Crusaders and becomes a member after being defended by Apple Bloom from bullies. Even when she becomes Nightmare Moon, the Crusaders still trust her, even when she reluctantly locks them in a dungeon.
In Pen Stroke's stories, Shining Armor falls victim to Nightmare Moon's mind control alongside most of the royal guards. When Nyx returns to normal and becomes good, Shining Armor is very judgmental toward her when he feels that she would hurt his sister. His discrimination causes him to lose the trust of his wife and sister. After a wild adventure and some other stuff, Shining Armor learns about Nyx's true self and that she was merely the focus of his own fear, the fear of losing his sister. After accepting Nyx as his new niece and apologizing to the rest of his family, he soon earns back his wife and sister's forgiveness.
Of all the enemies Nyx makes, Diamond Tiara and Silver Spoon (mostly the former) are the worst. In Past Sins, they cruelly ridicule her and send her in the Everfree Forest in revenge for asking questions before recess. Their hostile relationship intensifies after Nyx's Nightmare Moon experience.
In Pen Stroke's stories, Nightmare Moon defeats Trixie when the latter substitutes as the element of magic (since Twilight was absent due to being a prisoner in Nyx's castle), and keeps Trixie's hat and cape when she drops them. She imitates Trixie with the clothing, hoping to make someone laugh, with little success. When Twilight later finds out about the attempt, she admits that she finds it funny.
In the UPGverse, Nyx's bond with Twilight's closest friends seems to have grown. Spike's an older brother figure and Twilight's five best friends have become her honorary aunts, somehow also inheriting Pinkie Pie's bouncing ability to break the fourth wall and her rapping (learned fast from watching Pinkie rap), as well as Fluttershy's Stare and kindness to animals, Applejack's tree-bucking strength, athleticism and cooking, Rainbow Dash's weather manipulation (just need some practice first), her mother's magic skills and love of books, and Rarity's manners and gem-finding magic. Others' bonds, such as with Princess Celestia, Princess Luna, Princess Cadance and Shining Armor have also been portrayed as more amicable and familiar.
Most notable in JusSonic's series of stories is the new relationship with her (eventual) adoptive father, Ben Mare (short for Benjamin Maregillian). Nyx first met him when he was a Demon Pony and defended her against some timberwolves. Upon learning of the entire story of the Demon Pony's true identity, she gradually called Ben "daddy," since Ben Mare is Twilight's love interest, and later on, 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.
Another prominent third-party author that has written stories featuring Nyx is Reality Check. His Nyx stories start with the events surrounding Nyx's first Nightmare Night celebration after her creation. They have continued with the story of her reconciliation with Princess Cadence and Shining Armor and also some of her adventures in Ponyville School alongside the expanded Cutie Mark Crusaders.
The most recent of Reality Check's stories centered on Nyx is a story that has attempted to write Twilight Sparkle's ascension to an alicorn into the universe. It has also addressed the wider consequences of the "long night" caused by Nyx's short period as Nightmare Moon.
Another third-party author that's writing Nyx stories is MaskedPony. His stories start between after Equestria girls and before season four, where Nyx is with Rainbow Dash for a few months. His story follows Nyx from being known as the Nightmare Moon reborn, to a national hero.
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 lovable, 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.
Although, she is friends with the others. She can be a little bossy.
Acrophobia (fear of heights)
In The Wings Around the World Rally, Nyx is shown to have acrophobia. When she is riding with Yuna while training she gets vertigo and forces Yuna to fly back down. Later, after training, she then talks to Gator.
Relationship with Cheetor
In The Inquisitor's Revenge Part 1 & 2, After Yuna and Dusty found Nyx, Cheetor has been like a cousin to her as he joined her and the others to rescue Yuna from Trakeena and the Inquisitor, After rescuing Yuna, Princess Twilight Sparkle and Prince Flash Sentry granted Cheetor the privileged of being Nyx's guardian.
- Nyx is best friends with the Cutie Mark Crusaders and Snowdrop's direct descendant, Snow Pillow, the daughter of Queen Frostine.
- Nyx will appear in The Inquisitor's Revenge Part 1, and she'll be adopted by Prince Flash Sentry and Princess Twilight Sparkle in The Inquisitor's Revenge Part 2
- Nyx will meet Team Robot someday in near future.
- Nyx makes her appearance in The Foal, The Dark Alicorn, and The Sith
- In Thomas' Adventures in Transformers: Prime, Nyx will be adopted by Thomas and Twilight Sparkle.
- In some adventures, Nyx can be a little bossy.