Dæmon

The Dæmons were a race of powerful humanoids who profoundly affected humankind.

Biology
Dæmons could use their technology to change their size (TV: The Dæmons) and shape. (PROSE: T.memetics: A Morphology) Some appeared as satyr-like humanoids with reddish skin (HOMEVID: Dæmos Rising) or thick fur. These Dæmons often had sharp fangs, pointed ears, thick beards and long horns. Their legs were goat-like, with cloven hooves. (TV: The Dæmons)

During war, Dæmons would manifest in spatiodynamic bodies with vast wings, sweptback horns, and long necks, allowing them to glide through hyperspace with minimal effort. (PROSE: Interference - Book Two)

Some Dæmons had long, forked tails. (COMIC: Voyage to the Edge of the Universe)

Culture
Like a million other worlds "right across the universe", Dæmos had "the representation of the horned beast" in its myths and legends. The Beast claimed to be this "devil", along with all of the others in every religion which had such a figure. (TV: The Satan Pit)

The Third Doctor seemed to initially regard the Dæmons as not evil, but coldly logical and amoral, without sentiment, which Azal's actions would bear out. (TV: The Dæmons) They did only what they wished; self-sacrifice would have destroyed them. (PROSE: T.memeticus: A Morphology)

However, they also seemed to organise according to the nuclear family structure and care for their mates and children. (COMIC: Voyage to the Edge of the Universe) At a later stage of his life, the Seventh Doctor had made the acquaintance of a friendly Dæmon known as Bonjaxx and attended a birthday party in honour of him. (COMIC: Party Animals)

Technology
The Dæmons had technology indistinguishable from magic. They possessed matter-energy conversion technology which allowed them to shrink and grow things at will. They used this both to reduce the size of their ships and to manipulate their own size. They also had the ability to set up heat force fields which could disintegrate anything that moved into it. They could animate stone statues, as with the living gargoyle known as Bok. They could also discharge bolts of electricity.

A Dæmon could be psychically summoned by a special ritual, three of which were performed by. The ceremony required negative emotions such as fear, which could be provided by frightened humans. The more people that were involved and the more impressive the ritual, the more effective it was. Though these negative emotions could power the Dæmon, it also meant positive emotions could be dangerous. A Dæmon would only appear three times in response to these rituals.

Azal offered his powers to the Third Doctor when the Master, who desired them, had proved himself unworthy. This would suggest that they could give their powers if they chose to. (TV: The Dæmons)

Like their peers of the Osirian Court, their technology level qualified them for diplomatic and scientific relations with the Great Houses of the Homeworld. (PROSE: T.memeticus: A Morphology)

History
The Dæmons were an ancient race from Dæmos (TV: The Dæmons) who were powerful 150 million years BC. They were involved in the Millennium War and were one of the first species to strike back against the Mad Mind of Bophemeral, stopping it from using a dark-matter cloud in the Rift of Perseus. (PROSE: The Quantum Archangel) A Dæmon was one of the only beings to successfully travel to the edge of the universe. (COMIC: Voyage to the Edge of the Universe)

The Dæmons studied other races, treating the universe as their laboratory. The Third Doctor said that, circa 100,000 years BC, the Dæmons arrived on Earth and helped the humans to overcome the Neanderthals before trying to lead their evolution and development (TV: The Dæmons) in exchange for allegiance, as they often did with lesser species throughout the cosmos. Successful experiments were brought to be further cultivated in the Infernal Sphere, (PROSE: T.memeticus: A Morphology) but failed experiments like Atlantis were dispassionately destroyed. (TV: The Dæmons, PROSE: T.memeticus: A Morphology) According to one account, the Dæmons gave the Atlanteans the Crystal of Kronos, which later indirectly caused the destruction of Atlantis. (PROSE: The Quantum Archangel, TV: The Time Monster)

The Dæmons' ultimate goal was to cultivate a Child of Time, binding it to them so that they could leave Dæmos of their own free will. They interfered with human history to bring about such a creature. (PROSE: Child of Time)

The Dæmons were the only species to fight back against Kotturuh, and were wiped out in the attempt, save Azal. (PROSE: The Guide to the Dark Times)

The Third Doctor said that the Dæmons inspired myths of gods and demons and the image of powerful beings with horns, including the Egyptian god Khnum. They had been "coming and going ever since". Miss Hawthorne identified Azal as "the Devil". According to the Doctor, the Dæmons inspired the Greek civilisation, the Renaissance and the Industrial Revolution. (TV: The Dæmons)

Wesley Sparks claimed to have duelled Dæmons as one of his many achievements. (COMIC: Mortal Beloved)

In the 20th century, Azal called himself the last of the Dæmons. (TV: The Dæmons)

A Dæmon called Mastho took over monitorship of Earth after Azal's death. He was summoned to Earth at three different points of time. (PROSE: Child of Time) Firstly, he was summoned to 2004, where Kate Stewart and Douglas Cavendish stopped him. (HOMEVID: Dæmos Rising) The second time was in 1586, where the Sodality wished to harness his powers for their own benefit. Sensing that a Child of Time had been created by humans in this period, Mastho told the Grand Master and his Chancellor to eliminate all time channellers and time sensitives in order to lure it out into the open. He told them, once this was done, to summon him once more in 2586. At the site of the final summoning, Mastho called Maria, the Child of Time, to him. Maria refused to obey him and escaped into the time vortex, before committing suicide in 1949, the only way she knew of to stop Mastho's plans. Failing in his duty, Mastho self-destructed, consumed by the psionic energies he once commanded. (PROSE: Child of Time)

Near the time of the War in Heaven, the Dæmons were completely wiped out by some sort of massive cataclysmic event. It was believed to be a battle, as the Dæmon corpses were found in their battle form. Faction Paradox used the skeletal remains of the Dæmons as warships. (PROSE: Interference - Book Two) After Baphomet and Byzo tried to obtain metagenic field technology from the Great Houses in exchange for information about a crashed timeship on Earth, Lady Thelema used a handbell, an ancient book, and a candle to kill them. (PROSE: T.memeticus: A Morphology)

Minor references
The Dæmons were filed in the Fourth Doctor's memory files. (COMIC: Doctor Who and the Iron Legion)

When the Skith Leader scanned the Tenth Doctor's mind, Azal was among the alien creatures shown to him. (COMIC: The First)

Behind the scenes

 * The FASA The Doctor Who Role Playing Game "Sourcebook of Field Agents" states that Gallifreyan scientists have speculated that the Great Vampires are an offshoot of the Dæmons.