Dæmon

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

Biology
Dæmons were satyr-like humanoids with reddish skin and thick fur. They had sharp fangs, pointed ears, thick beards and long horns. Their legs were goat-like, with cloven hooves. They were around thirty feet tall naturally, but they could use their technology to change their size. (DW: The Dæmons) Some of this species, like the traditional picture of a demon, possessed long, forked tails. (DWM: Voyage to the Edge of the Universe)

Dæmons of the military were modified, giving them vast wings, spatiodynamic bodies, sweptback horns and long necks, allowing them to glide through hyperspace with minimal of effort. (EDA: Interference - Book Two)

Culture
The Dæmons had a horned god of evil. Though this may simply seem like a personified god, the Tenth Doctor surmised that it was actually inspired by the Beast. (DW: The Satan Pit)

The Third Doctor seemed to initially regard them as not evil, but coldly logical and amoral, without sentiment, which Azal's actions would bear out. (DW: The Dæmons)

However, they also seemed to organise according to the nuclear family structure and care for their mates and children. (DWM: 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. (DWM: 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 increase their own size (which could range from microscopically small to thirty feet tall) and to reduce the size of their ships in order to hide them. 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 Master. 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. (DW: The Dæmons)

History
The Dæmons were an ancient race from Dæmos (DW: 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. (PDA: The Quantum Archangel)

The Dæmons studied other races, treating the universe as their laboratory. (DW: The Dæmons) Using a spacecraft, one Dæmon travelled to the edge of the physical universe, a feat achieved by few other beings. (DWM: Voyage to the Edge of the Universe)

Circa 100,000 years ago, they arrived on Earth, helped the humans overcome the Neanderthals and then tried to lead their evolution and development. They were known to have destroyed failed experiments, such as Atlantis. (DW: The Dæmons) The Dæmons gave the Atlanteans the Crystal of Kronos, which later indirectly caused the destruction of Atlantis. (PDA: The Quantum Archangel, DW: The Time Monster)

The Dæmons inspired myths of gods and demons and the image of powerful beings with horns, including the Egyptian Khnum, as well as the Devil. (DW: The Dæmons)

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

By the 20th century all the Dæmons, save Azal, had died. (DW: The Dæmons)

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. (EDA: Interference - Book Two)

In the 7th season of the new series, there is an asteroid called Dæmons Run, named after one of the legend surrouinding The Doctor, talking about a great battle.

Behind the scenes

 * When the then-current production team of Doctor Who created the Dæmons for the story of the same name, they had no idea that later stories would introduce true demons into the Doctor Who universe. Equally, the next year in the story The Time Monster they contradicted the scripts reference to the Dæmon having destroyed Atlantis, substituting a completely different explanation.
 * In the Doctor Who spin-off, Daemos Rising, and the Time Hunter book, Child of Time, a living Daemon called Mastho was summoned by the sodality, but like Azal, he too was destroyed. It was unsure if this was either before the cataclysmic event or after, or if there were any other surviving Daemons.