Dæmons were satyr-like humanoids with reddish skin and thick fur. They had sharp fangs, pointed ears, a thick beard and long horns. Their legs were goat-like with cloven hooves. They were around thirty feet tall naturally, but they could change their size with their technology. Some of this species, like the traditional picture of a demon, possessed long, forked tails.
Dæmons of the military were modified, giving them vast wings, spatiodynamic bodies, sweptback horns and long necks, allowing them to glide through hyperspace with the minimum of effort.
The Dæmons had a horned god of evil. Though this may simply seem like a personified god, the Doctor surmises that it was actually inspired by the Beast.
The Doctor seemed to initially regard them as not evil, but coldly logical and amoral, without sentiment, which Azal's actions would bear out.
However, they also seemed to organize according to the nuclear family structure and care for their mates and children. At a later stage of his life the Doctor had made the acquaintance of a friendly Dæmon known as Bonjaxx and attended a birthday party in honour of him.
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 to increase both their own size (which could range from microscopically small or to thirty feet tall) and to reduce the size of their ship 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 also 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 require negative emotions such as fear, which can be provided by frightened Humans. The more people that are involved and the more impressive the ritual, the more effective it is. Though these negative emotions can power the Dæmon, it also means positive emotions can be dangerous. A Dæmon will only appear three times to these rituals.
Azal offered his powers to the Doctor when the Master, who desired them, had proved himself unworthy. This would suggest that they can give their powers if they choose to.
The Dæmons were an ancient race from Dæmos who were powerful 150 million years ago. 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.
The Dæmons studied other races, treating the universe as their laboratory. Using a spacecraft, one Dæmon travelled to the edge of the physical universe, a feat achieved by few other beings.
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 are also known to have destroyed failed experiments, such as Atlantis. They are also believed to have created the Immortals. The Dæmons also gave the Atlanteans the Kronos Crystal, which later indirectly caused the destruction of Atlantis.
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.
Wesley Sparks claimed to have duelled Dæmons as one of his many achievements.
By the 20th century all the Dæmons, save Azal, had died.
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.