Demon

Demons was a broad term used by some cultures to refer to entities that were perceived as supernatural, but were not gods or spirits. Demons were commonly depicted as malevolent and as humanoids with horns, although the term could be applied more broadly on both accounts.

Although some individuals were referred to exclusively as "demons", the term was often misapplied to entities of more mundane origins but whose powers made them appear nearly godlike to "lesser" beings — including the Dæmons of Dæmos and a variety of alien powers who tried to conquer the Earth in pre-modern times.

As only or main identifier
Demons were "manifestations of total power in essence". They could be summoned through magical rituals of varying complexity, but needed to be bound close to obey their summoners" will; seeking their freedom, they would inevitably "cheat and lie" to trick the wizards who had summoned them into setting them free. (PROSE: Battlefield) One such demon was the Destroyer, also known as "the Lord of Darkness" and "Eater of Worlds", was an entity whom Morgaine, a sorceress, had the ability to summon and bind to her service. He was vulnerable to silver, which could bind him or even kill him, and had the appearance of a hulking blue-skinned humanoid with horns. (TV: Battlefield) The Seventh Doctor believed him to be "one of the 7,405,926 demons on the Talmudic table", more precisely a member of "what Ace would call the Top Ten". (PROSE: Battlefield)

According to Ida Scott, the scriptures of the Veltino on the subject of Krop Tor spoke of a "mighty demon" to whom the planet was poison. The conventional interpretation of the legend was that the "demon" was a mythologised version of the black hole, but the Tenth Doctor and Rose Tyler discovered that there was actually an entity trapped in a black pit on Krop Tor, known as the Beast, who claimed to be the Devil and an ageless embodiment of all evil. The Beast's physical form was a huge red-skinned humanoid with horns and a skull-like face, but he was capable of projecting his mind into other bodies. (TV: The Satan Pit/The Impossible Planet) The Beast's son Abaddon was an ancient being sealed behind the Cardiff Rift. (TV: End of Days) Abaddon also fought another demon who resembled him, Pwccm. (PROSE: The Twilight Streets)

An entity from a Shadow Dimension was hired by the alien Hornets to trap the Doctor and drain his mind of all his knowledge. The entity's appearance and frequent need to drain the life force of humans to sustain itself made the Fourth Doctor call it a demon. (AUDIO: Demon Quest)

Cultural manifestations
One Manussan tradition included people playing the part of "attendant demons" at a festival of initiation; people who misstepped during the festival would have water tipped on their head by the "demon" as a mock "punishment". (TV: Snakedance)

The Captain, apparent ruler of Zanak, frequently swore by the sky demon. (TV: The Pirate Planet)

Justine believed that Vincent Wheaton's powers were the result of a demon inside him, although Ace scorned this belief. (PROSE: Cat's Cradle: Warhead)

Misidentifications
Marchosias, a gestalt manifestation of the Psionovores, took the form of an eight-foot tall bright red demon when "summoned" as an agent of Lucifer by Brigham Elisha Dashwood III and the Hellfire Club. (AUDIO: Minuet in Hell)

The Dæmons were a race of advanced aliens from Dæmos. During their travels on Earth, they probably influenced the mythology of the native humans. (TV: The Dæmons) They themselves had a figure in their mythology similar to the Beast. (TV: The Satan Pit)

In the court of King John of England (actually Kamelion), the Fifth Doctor, Tegan and Turlough found themselves mistaken for demons due to their "magical transportation". (TV: The King's Demons) was seen to humans as a demon when his form was comprised solely of energy. He used demonic powers such as possession to find new hosts for his dying form. (PROSE: Prologue)

After discovering that they really existed, Victor Lang reasoned that vampires must be a kind of demon. (PROSE: Goth Opera)

The Arthurian wizards Merlin and Ganeida were said to be "half-demon" on their father's side; this heritage granted them an augmented lifespan and magical powers. Indeed, Ganeida identified herself as "half-demon" explicitly. However, Agamya Akhtar suggested that "demon" may have been the term used by the residents of Camelot for an alien of some kind; Merlin hinted to his sister that this was the case. (PROSE: A Honeycomb of Souls)

Within the Shadow World, the Veritas, a text which was a record of the truth of this universe (that it was an artificial simulation), attributed the creation of it to an "evil demon" who wanted to conquer the world. In reality the simulation was not created by one being but by the Monks, powerful beings who needed to make deals with mortals, obtaining their explicit and sincere consent, before they took over a planet. (TV: Extremis, The Pyramid at the End of the World)