Fairy

The Fair Folk or Fae, also known as the Sidhe, and most commonly called fairies (though Jack Harkness thought the name was improper), were magical, humanoid entities who existed alongside humanity on Earth on some level of reality that overlapped with the mortal realm. They were sometimes equated with elves. Though a fixture of legends and imaginary stories, as evidenced by the term "fairy tales", many accounts agreed that these beings actually existed in the Doctor's universe. The Twelfth Doctor once claimed that they had a scientific name, Homo fata vulgaris.

Nature
The Twelfth Doctor described the Fair Folk as originating in a realm known as the Invisible which existed parallel to "the Visible" that mortals ordinarily perceived. According to the Eighth Doctor, the Sidhe's "magic" was primarily rooted in the fact that they could perceive and act through "all eleven dimensions" instead of humanity's four. Thus, their realm overlapped with the mortal realm humans inhabited, and they could manifest therein at will, but they would usually be imperceptible to humans due to being "out of phase" with them. Their three-dimensional forms were therefore highly malleable and changeable; a single fairy might appear alternatively as a small figure with clearly inhuman features, as almost human except for pointed ears, or as more monstrous or ethereal forms such as patterns of lights or moving, solid shadows. This was analogus to the different ways a three-dimensional object might look from a two-dimensional perspective. (PROSE: ) One account showed fairies manifesting both in a small, glowing humanoid form with butterfly-like wings, and in a human-sized, green-skinned, more starkly inhuman and threatening form which still retained wings. (TV: Small Worlds) Even when manifesting in forms with slim and fragile-looking builds, fairies were not only agile, but immensely strong and capable of overpowering fully-grown men with ease. (TV: Small Worlds, PROSE: Autumn Mist)

Also as a result of their dimensional out-of-phaseness, (PROSE: Autumn Mist) they could travel through time without need of a time machine, living non-linear lives; (PROSE:, TV: Small Worlds) a picture of Jasmine Pierce, who became a fairy in the 21st century, was seen in her fairy form in 1917. (TV: Small Worlds)

Fairies "controlled the elements of Fire, Earth, Air and Water", making them highly dangerous opponents for mere humans. They could produce localised, powerful storms and gales. Not bound by normal physical laws, they could choose whether a camera could take their pictures. They made themselves invisible to closed circuit television cameras on one occasion. On another, they let their photographs be taken. They could kill their victims by "stealing their breath", asphyxiating them by clogging their throats with rose petals, drowning them via rainstorms or using gales to draw all oxygen away from the victim. (TV: Small Worlds)

However, they had a great weakness to metal, particularly iron, as they interfered with their natural electromagnetism; a fairy trapped in an iron cage would be trapped, and prolonged contact with metal would kill them. (PROSE: )

Fairies could turn humans into more of their own, granting them immortality in the process and the ability to perceive the higher dimensions. (PROSE:, TV: Small Worlds) One account even suggested that all fairies had once been human children, taken out of time; the "Chosen Ones". (TV: Small Worlds)

Culture
The Sidhe were organised in clans, such as the Leannain Sidhe, who kept their bloodlines separate, but all answered to the authority of the Fairy Queen, Titania. She was eternal, and there was only ever one Fairy Queen, but she could have multiple "aspects"; if one was destroyed or was deemed unfit to fulfill her duties, she would be replaced. Her consort was the Amadan na Briona. The fairies' culture and spirituality paid homage to the duality of chaos and order, and, just as the Queen embodied Order, the Amadan embodied Chaos; they could not interfere in one another's affairs. The marriage was usually a purely political one, though at least one aspect of Oberon, the Amadan, was genuinely possessive of Titania and resented her countless affairs with mortals and other Sidhe. (PROSE: )

The Sidhe did not generally interfere with human affairs on a wide scale, (PROSE:, TV: Small Worlds) owing to an ancient pact between the Sidhe and early humans; though they still honoured the terms of this pact, by the 20th century the Sidhe were aware that humanity had largely forgotten the old agreements, and did not generally exact punishment when mortals broke its terms out of ignorance. (PROSE: )

According to the account which depicted young Chosen Ones as the apparent sole origin for new fairies, they were fiercely protective of these Chosen Ones, and would begin to interfere if humans tried to prevent the Chosen Ones from being taken. (TV: Small Worlds)

The Sidhe were aware of the Doctor, whom they called the Evergreen Man. Though they abided by the Laws of Time, Titania claimed that the Evergreen Man was the only "Lord of Time" whose Lordship over time she actually acknowledged. (PROSE: )

Origins
Jack Harkness claimed that fairies came "from the dawn of time" and that it was impossible for humans to fully comprehend their nature, floridly describing them as "something out of the corner of your eye with a touch of myth, a touch of the spirit world, a touch of reality, all jumbled together". (TV: Small Worlds) The Eighth Doctor speculated that the Sidhe were "psychomaterial constructs from a parallel evolutionary path" relative to humanity, (PROSE: Autumn Mist) and indeed, the Twelfth Doctor gave their scientific designation as Homo fata vulgaris. (PROSE: The Shining Man) Jack once mused that he wondered if the fairies were "part Mara", whom he described to Toshiko Sato as a "kind of malignant wraith" whose name was the origin of the word nightmare. (TV: Small Worlds)

Throughout human history
When the Earth was young, the veil between the Invisible and the Visible was thinner, allowing fairies to cross the divide and wreak havoc in the human dimension. They frequently attacked Huckensall village. The people in the village protected themselves by building the doors in the village from the wood of a rowan tree. In 1654, a Fairy Finder came to the village and rounded up the Fae, burying them deep in the ground. (PROSE: The Shining Man)

A fairly large group of Fairies lived in Roundstone Wood, outside of Cardiff, and had various standing stone circles arranged in it. Many avoided the woods throughout history, thinking it a cursed place; even the invading Roman Empire steered clear of the forest and it was considered bad luck to even walk through them or collect wood from the forest. Even into the 21st century the woods remained unharmed and housing developments hugged its very edge but did not move any further in. (TV: Small Worlds)

20th century
In 1944, in the Ardennes, the American Colonel Allen Lewis sought to continue experiments with dimensional phasing which he had begun as the Philadelphia Experiment the year prior. He managed to open a rift over the Schnee Eifel, which made shifting into the Sidhe realm easier in the surrounding area, and began experimenting with the possibility of outfitting tanks that could pass into the faerie lands. He contacted his Nazi counterpart Jurgen Leitz, also a researcher of the occult, who referred to the Sidhe as "Light and Dark Elves, and they worked together in secret even as their forces clashed, having agreed that the real War would be between the West and the Soviet Union, and they needed to start preparing for it now, whatever the outcome of World War II.

Because of the rift, the violence of World War II began leaking into the Sidhe realm, with every bombing and shelling causing the realm's beauty to decay on every layer of perception the fae could perceive. Queen Titania decided to decree that so long as the realms bled into one another, human lands overlapping with the present location of her court would count as part of her domain; as a result, she claimed all who died on the battlefields as chosen, and intercepted their souls as they died, before healing them and regenerating them into Sidhe themselves. Rumours of the mysterious beings who snatched the bodies of the only-just-dead, appearing out of a mysterious mist, began to spread around the men, with the Polish-born Wiesniewski interpreting them as Leshy (woodland elementals of Polish legend) and the Hopi-descended Daniel Bearclaw referring to them as Kashinas, or "cloud people". Meanwhile, her chaotic consort Oberon decided to help Colonel Ellis, not in service to any wider ambition but simply to spread mischief and, secretly, to see how far he could push his rule-breaking before someone would stop him.

After the Eighth Doctor's TARDIS materialised not far from the rift, he and his companions Sam Jones and Fitz Kreiner quicky became separated. While the Doctor made it to an American base and began investigating, spurred on by a cryptic encounter with Titania (who could not openly interfere, but hoped he would oppose Oberon's schemes), Fitz unwillingly found himself undercover at the Nazi encampment, where he discovered Leitz's experiments and freed his Sidhe prisoner. Sam, meanwhile, was found by another American squad and was on her way to join the Doctor when their entire column was halted and then slaughtered in cold blood by Nazis. After dying, Sam was rescued by the Fae and regenerated into a fairy herself, being welcomed into the Sidhe realm by Galastel. Eventually, the Doctor was reunited with his companions and they were able to close the Rift, also destroying Oberon's current aspect in the process — though the Doctor was confident Titania would find herself a new Amadan almost instantly. Sam ended up requesting to be returned to her basic human state, losing her perception of the higher dimensions. (PROSE: )

The Seventh Doctor claimed in 1957 that many so-called UFO sightings in the 20th century were actually down to the Sidhe. (PROSE: )

21st century
In 2002, Griffin spotted a Sidhe in San Francisco. (PROSE: Unnatural History)

In the mid-to-late 2000s, Torchwood Three investigated events surrounding a girl called Jasmine Pierce, whom the fairies had selected as a Chosen One. They brutally killed Mark Goodson, a paedophile who had attempted to abduct Jasmine, and also exacted supernatural retribution on bullies who'd made fun of her. Torchwood attempted to stop the fairies from taking Jasmine, but they refused, threatening to freeze the entire Earth (which they said would be of little concern to them, as they could simply confine themselves to other eras of time). Jack Harkness, who had encountered the fae before and knew of their power, relented, and Jasmine herself thanked him as she went to join the fairies in the Lost Lands. Torchwood would go on to realise that an older Jasmine she was one of the fairies depicted in the Cottingley fairy photos. (TV: Small Worlds)

In 2017, the Shining Man craze on the internet led to a general sense of fear among the public, and the barriers between the Invisible and the Visible began to weaken, allowing fairies to interact with humans again. (PROSE: The Shining Man)

Undated events
The Doctor encountered fairies alongside Madame Vastra during his ninth (or earlier) incarnation. (COMIC: The Lost Dimension)

Cultural legacy
Titania appeared in William Shakespeare's play A Midsummer Night's Dream. (PROSE: Slow Decay)

In non-valid sources
Mavis Cruet, a fairy, once flew through Festive Road, before being chased by Gnasher. (TV: Future Generations)