Eternal

The Eternals were elemental beings of immense power but limited creativity, who used the thoughts and emotions of so-called "Ephemerals" for their own ends as their own minds were used up. Although they had variable physical forms, they were often considered a species in their own right.

Characteristics
According to some legends, when a child of the Age of Reason died, her guilty spirit could become lost in Null-Space where its phobias and anxieties fed on less vindictive spirits. These spirits may have eventually grown into the Eternals. (PROSE: Christmas on a Rational Planet) Death claimed that she and her fellow Menti Celesti were "the dreams of Time Lords", and that they "leak out across the universe", only occasionally being given physical form by entities such as the Timewyrm. (PROSE: Timewyrm: Revelation)

The Eternals were among the "Old Ones", species which had risen to prominence in the Dark Times only to fall from grace somewhat in later epochs. (PROSE: The Knight, The Fool and The Dead) River Song wrote that the Eternals had been "young" for "a millennium or two" after the Big Bang. (PROSE: The Guide to the Dark Times)

The amoral Eternals made it their principal aim in life to amuse themselves, using Ephemerals as their toys. (TV: Enlightenment) In the course of their games, the Eternals would sometimes pose as gods and doom whole worlds in the process. (COMIC: Uninvited Guest) These games were started by the Guardians of Time, whom the Eternals greatly respected as leaders and called "Enlighteners." Though they were related to the Chronovores, they looked down on them as vicious predators. The Ancient Covenant prohibited a union between the two races. However, the great Kronos arose from one such union. (PROSE: The Quantum Archangel)

Through somewhat obscure means, as a punishment, the Seventh Doctor used the force of the Time Vortex itself to bring the particularly callous Prospero and his social circle into the domain of Time, which would indicate that under some circumstances they could transition from Eternal to Ephemeral. (COMIC: Uninvited Guest)

Powers
The Fifth Doctor called Eternals parasites for using Ephemerals for their creativity. Eternals could pull any being from any planet in any time, and use them. The Eternals dwelt in the domain of Eternity, rather than the smaller one of Time. This meant they were unaffected by Time and thus unaging. They could even freeze Ephemerals in time. Eternals could also use telepathy and create objects from the memories of Ephemerals. Their powers were not limitless, however, as they could not read minds from great distance or from strong minds. Eternals could also concentrate and channel great power (in one case, power from the Black Guardian), but only if there was a focal point. Eternals could not be destroyed, merely transferred, (TV: Enlightenment) due to their existence as disembodied energy matrices. (PROSE: Goth Opera)

Eternals were time sensitives, (PROSE: The Crystal Bucephalus) trans-dimensional beings that could move through time and space at will, from planet to planet. They could cloud people's minds and make people accept them, influence their behaviour, and make them forget they were ever there. In any direct confrontation they could draw power from the strength of their opponent. Some of the weaker Eternals could only move things, like a poltergeist. (PROSE: Blood Harvest)

They could also appear in dreams and control them, (PROSE: Love and War, Set Piece) usually assuming the form of humanoids, but could change their shape into different individuals. They were able to control time and such claimed to control fate and destiny. (AUDIO: Master)

History
The Eternals, as their name suggests, had lifespans that lasted forever, therefore a history of their race would be hard to find. However, the first event that they are known to have been involved in was to banish the Carrionites and Hervoken into the Deep Darkness during the Dark Times of the Universe. (TV: The Shakespeare Code, PROSE: Forever Autumn) During the Dark Times, the Eternals sacrificed their ability to lead their own lives in the process, starting their reliance on Ephemerals, to escape the Kotturuh. (PROSE: The Guide to the Dark Times) On Earth, they could be recognised as gods, angels, earth spirits, devils, demons, ghouls, and poltergeists. (PROSE: Blood Harvest)

Some of the Eternals passed themselves off as Gallifreyan deities, the Menti Celesti, such as Time, Pain and Death, a development which Lady President Romana II found distressing. (PROSE: Happy Endings) Not all Eternals did this, and until he met the Fifth Doctor and probed his mind, the Eternal known as Striker professed that he had never heard of the Time Lords. (TV: Enlightenment)

A temporal skirmish opposed the Eternals to the Halldons, but an end was brought to it by the Time Lords, in one of the very few time wars they would fight prior to their great War with the Daleks, (PROSE: Meet the Doctor) the, by a dissenting account of their history that disagreed with the stories of Yarvelling or Davros, rebellious creations of the Halldons. (PROSE: We are the Daleks!)

At some point, the Guardians of Time set up a race for a group of Eternals, with Enlightenment being the prize. Groups of Eternals copied ships from various points in Earth's history, fitted them with ion drives and sails to catch solar winds and kidnapped humans to run the ships. Though Captain Wrack almost won the race by destroying the ships of her enemies, the Fifth Doctor managed to push her off her ship and bring it to the Guardians of Time. (TV: Enlightenment)

The Eternals were aware of the legends of the Highest Science. (PROSE: The Highest Science)

The Monk used the blood of the Eternal Vain Beauty he had met in Paris to trap Artemis the Chronovore. (PROSE: No Future)

In 2604, Bernice Summerfield became caught up in the conflict between the Eternals Raymond Hardy and Barron. (AUDIO: The Heart's Desire)

During the Last Great Time War, the Eternals finally forsook the universe, (PROSE: A Brief History of Time Lords) abandoning their "hallowed halls" as they "despaired" of this reality. (PROSE: Meet the Doctor) However, long after the War had ended from his perspective, the Tenth Doctor still warned the Hervoken to beware of attracting the Eternals' attention. (PROSE: Forever Autumn)

Two Eternals, (PROSE: The Guide to the Dark Times) Zellin and Rakaya, once descended from a "higher plane". Finding two inhabited planets in the same solar system, they made a bet as to who could destroy a planet first to amuse themselves. The inhabitants of the planets went to war with their own people and eventually each other. The inhabitants of the planets eventually became aware of Zellin and Rakaya's influence and managed to imprison Rakaya, but Zellin managed to escape before he could be imprisoned and sought to liberate her for millennia. The efforts of the Thirteenth Doctor and Team TARDIS, with the additional help of Tahira, eventually foiled his plans and forced him into the same prison as Rakaya. (TV: Can You Hear Me?)

Other information
The Eternals referred to the Void as "the Howling". (TV: Army of Ghosts)

Sebastiene claimed to have hunted down an Eternal. (PROSE: The Doctor Trap)

Behind the scenes

 * The Dalek Handbook identifies the Eternals as one of the Higher Species due to their involvement with the Time War.