Nightmare was born in a Clan that has long been gone, to her parents Lunarstar and Hawkstorm. Her sisters were Dreamkit, Skykiy, and Berrykit. Daykit was always jealous of her sisters because they always seemed to exclude her from everything. Daykit took an early interest in herbs and healing, and loved the thought of being an influential cat who others had to look to - she also loved the fact that she could hold the lives of other cats in her own paws. So, she followed the path of a medicine cat as Daypaw, while Dreampaw, Skypaw, and Berrypaw became warrior apprentices.

As an apprentice, Daypaw stumbled across some deathberries, unknowing what they were at first. She fed them to a kit, not wanting to eat them herself, so she knew what they were. The kit, obviously, started convulsing from the berries. Daykit immediately ran back to camp to get Midnightstream, the medicine cat, and brought the three-legged cat to where the kit was, now dead. Midnightstream, unaware that Daykit had fed the berries to the kit, informed her what they were and instructed her to tell the kits to staya way from any bright red berries. Daypaw, however, loved the feeling that the lives of cats were in her paws. She used it to her advantage.

Later on, her sisters were made into warriors; Dreamwing, Skywing, and Berrywing. Again, Daypaw was jealous. She was made into Daydream. Daydream was very envious of her sisters and how close they were. Hawkstorm, their father,seemed to take an interest in the three of them moreso than her, as if he were prouder of his warrior children rather than his medicine cat child.

Soon later, Midnightstream died, making Daydream the sole medicine cat of her clan. She then had an idea. She went to her father, Hawkstorm, and informed him of a "message from their ancestors", stating that Hawkstorm must leave the Clan and go on a mission to save his Clan from disaster. Hawkstorm took the message and left right away, ordering Daydream to inform the others of where he had gone. Daydream didn't say a word. She knew she might as well have sent him to his death. Everyone was distraught, especially Skywing when Lunarstar ordered everybody to give up the search and pronounced him dead. Skywing left to look for him anyway, leaving Dreamwing and Berrywing by themselves, who grew closer to a couple older warriors who grieved with them. Daydream was even more distraught and envious, because SHE wanted to be the one to grieve with her two birth sisters.

Soon later, the Clan's deputy, Brightfang, retired and became an elder, leaving Lunarstar to name Dreamwing as deputy. Dreamwing then proceeded to request to change her name to Sootclaw, which was approved. Daydream also changed her name to Nightdream. She was still very envious that her sister had gotten a respected position as well and still managed to be closer to the other siblings.

She decided enough was enough. One day, she left the camp late at night. She came across a bonfire that the humans had created. She let them spot her, and one of them took a branch on fire and threw it at her. The fire branch ended up hitting something flammable, setting more of the forest on fire. Nightdream turned tail and fled, not even bothering to warn the rest of the Clan of the fire. Had she done so, the Clan might have survived. But she did not care.

Moons later, she came across a new Clan, led by Sootclaw, now Sootstar. Nightdream discovered that Sootstar had a litter of kits. Nightdream left, deciding to change her name to Nightmare, as she vowed to be her sister's worst nightmare yet. This Clan soon disbanded and turned into a different Clan led by Lunastar, Sootstar's daughter, with Sootstar nowhere to be found.

Soon later she met up with an old cat from her original Clan, Abyss, who seemed to share her same interests. The two of them joined together and traveled long and far. They journeyed to a place that smelled of large cat groups. They fell in love and had two kits, and left them to be found by one of the groups. When the kits grow up, Nightmare and Abyss will take them away. They will prowl around the territory and wreak havoc on whoever dares cross them...

Nightmare is very influential and manipulative, and she uses those aspects to her advantage. She manipulates other cats into doing her dirty work and influences cats to follow her because what she believes must be right. She puts herself first, always, and only does things if it benefits her. She refuses to take no for an answer.

Nightmare is a pure black cat with glaring amber eyes that can sometimes look blood red.