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.

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)

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 behavior, 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 and such claimed to control fate and destiny. (AUDIO: Master)

History
The Eternals were, as their name suggests, eternal, 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) 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 the Gallifreyan deities 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.

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)

The Tenth Doctor warned the Hervoken to beware of attracting the Eternals' attention. (PROSE: Forever Autumn)

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

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)

Adam, an alien who infiltrated Torchwood Three, showed powers similar to those of an Eternal, and also said that he had escaped from the Void. (TV: Adam)

Behind the scenes

 * According to Doctor Who Annual 2006, a non-narrative source, there had been a time war between the Halldons and the Eternals, and the Time Lords had intervened to stop it. It also said that the Eternals watched the events of the Last Great Time War, and upon seeing all the devastation and the death, fled their "hallowed halls" and were never to be seen again.