Demons are powerful beings of darkness. In the series, they differ greatly in their forms, some can have heads like wolves, or lions, or insects, or humans, others can have the body of a snake, or a moose. The combinations are endless. Demons vary greatly in appearance but all look like creatures of terror, fear and nightmares and are usually very ugly and fearsome.
In past ages, demons almost succeeded in taking over the world, which is why many were locked away in Zzyzx, the Great Demon Prison.
A demon named Bahumat almost overthrew Fablehaven after he was released from his prison at the Forgotten Chapel by Muriel Taggert, but both were promptly recaptured by a group of enlarged Fairies summoned by Kendra.Seth was bitten by the inert statue form of the demon Olloch the Glutton. Olloch almost devoured Seth in the Valley of the Four Hills, but Seth managed to survive using Coulter's magical cocoon.
Fablehaven almost fell because of a shadow plague that turned light creatures into dark creatures. It was later found out that the plague was started by the demon Kurisock with the help of Ephira. The plague was stopped by Lena using a magical stone retrieved from the Fairy Queen.
The demon prison Zzyzx is opened by Graulas and Nagi Luna. The Knights of the Dawn delay them long enough for the secret plan formed by Bracken and the founders of Zzyzx to work, turning the old Fairy Realm into the new Demon Prison.
Kendra speaks with a demon named Jubaya, who hides in a sludge pit inside the new Fairy Realm. She requests that Seth should seek out Talizar, Batoosa, and/or Vez Radim. Later, Seth unintentionally finds and denies aid from Talizar in the Path of Dreams.
- Bahumat - imprisoned, with Muriel, underground in the place where the Forgotten Chapel once stood, after Kendra Sorenson led the Fairies in battle to defeat the Demon.
- Batoosa - a demon that Jubaya said could help Seth unlock new Shadow Charmer abilities.
- Brogo - son of Gorgrog, a gigantic demon, and one of first imprisoned in Zzyzx. He was rumored to be the strongest demon in history. He was severely injured by Celebrant during the Battle of Zzyzx.
- Din Bidor - a wolf-like Demon, fear and darkness increase his size. One of the first Demons to be released from Zzyzx, fate unknown.
- Gorgrog - the demon king for much of the events of the Fablehaven Series, imprisoned in Zzyzx ages ago until he was slain by Kendra Sorenson with the help of the Fairy Queen.
- Graulas - a very old demon, who was hibernating in his cave at Fablehaven for centuries. After Seth escaped from Living Mirage and healed Graulas with the Sands of Sanctity, the demon proceeded to destroy the founding treaty of Fablehaven, overthrow the Sphinx as leader of the Society, and helped open Zzyzx with Nagi Luna. He was later killed by Seth Sorenson wielding Vasilis at the beginning of the battle of Zzyzx.
- Humbuggle - the powerful, mischievous demon who usually takes the form of a dwarf who overtook Stormguard Castle in order to create the contest for the Wizenstone. He is currently home in his castle at Titan Valley.
- Ixyria - a mentor to Witches and Hags. One of the first demons to be released from Zzyzx, fate unknown.
- Jubaya - a humanoid demon with a head that appears like the charred face of a praying mantis. Jubaya was among the few demons who did not leave Zzyzx when it was opened, staying behind in her pit. When she sensed Kendra Sorenson enter Zzyzx after it was taken over by the Fairies to transform it into the new Fairy Realm, she offered to be willingly moved to the new demon prison in exchange for the chance to speak with Kendra. She offers interesting information about the Dragons and the names of three other demons, Talizar, Batoosa, and Vez Radim who Kendra's brother Seth Sorenson may seek out to hone his skills as a Shadow Charmer.
- Kurisock - was eradicated by Kendra with the help of Lena, at the end of the Battle of Kurisock's Domain who sacrificed herself by touching the corrupted nail with the talisman made by the Fairy Queen.
- Lycerna - a reptilian demon who wreaked havoc at the Rio Branco preserve after taking it over and destroying it's founding treaty.
- Nagi Luna - clairvoyant demon imprisoned in the Quiet Box in Living Mirage. A mentor to the Sphinx she was the one who helped him become a Shadow Charmer and take over the secret preserve. After betraying the Sphinx during the final stages of the Zzyzx Crisis, she and Graulas took control of the Society and released the demons imprisoned in Zzyzx during the Battle of Shoreless Isle. She was slain by Seth Sorenson
- Olloch the Glutton - immobile as a small frog statue until somebody feeds him and restarts his unending hunger. He returns to his dormant state after he has devoured the person who initially fed him.
- Orogoro - the eldest son of Gorgrog. He looks like his father, but with moose-like antlers. Orogoro was injured by Gilgarol during the Battle of Zzyzx. After his father was slain, he became the new demon king after claiming his fallen father's crown.
- Talizar - a cloaked demon with a face like a wolf, glowing red eyes, rat-like hands and feet. Seth encounters Talizar in the Path of Dreams, where he expresses an interest in Seth for his status as a Shadow Charmer. He offers to become his new mentor but Seth refuses. Although denied, Talizar simply shrugs it off and tells Seth his offer is still open if he ever changes his mind and if so to seek him out in the waking world.
- Vez Radim - a demon that Jubaya said could help Seth unlock new Shadow Charmer abilities.
- Zorat the Plagueman - tall, thin, humanoid demon covered with sores. One of the first demons to be released from Zzyzx. His fate is unknown.