Eternal (Doctor Who)
The Eternals are a race of cosmic beings first introduced in the Doctor Who TV adventure Enlightenment. One Eternal who called himself Striker explained to the Doctor that he and his people lived outside of time, in the realm of eternity. They considered the mortal inhabitants of the universe to be "Ephemerals", even the Time Lords of the planet Gallifrey. Striker seemed completely unaware of the existence of the Time Lords before meeting the Doctor.
In the Enlightenment serial, the Doctor eventually realized that although the Eternals were powerful enough to manipulate matter, creating objects out of thin air, and read minds effortlessly through telepathy, they lacked imagination and creativity. Thus, they actually depended on "Ephemerals" to keep them active and prevent them from withering away in boredom, and without them the Eternals had no purpose; on one occasion the Doctor manages to outwit an Eternal attempt to destroy a rival ship by throwing the explosive off the ship, noting that the Eternals couldn't have accomplished that because they lacked the imagination to think of such an action. This dependence was not something they liked admitting to, however, and they made boasts several times of how they could manipulate the reality around them through sheer force of will. Despite this great power, they deferred to the Guardians of Time, specifically the White Guardian and the Black Guardian who offered the Eternals "enlightenment"- complete knowledge of good and evil- if they won a cosmic race. The Eternals captured many Ephemerals to win the race for them, hence accidentally bringing about the attention of the Doctor who immediately saw them as a threat.
Although the Eternals were never again seen in the TV series, they have been mentioned in the new TV series that began in 2005. In the episode "Army of Ghosts," the Doctor mentioned the existence of a "nowhere place" that exists between parallel universes and alternate timelines, saying that his people called it the Void while "the Eternals called it the Howling."
In the Doctor Who Magazine story "Uninvited Guests", the Doctor used the force of the Time Vortex itself to punish the evil Eternal called Lord Prospero by bringing him into "the domain of Time." So it seems that under the right circumstances, an Eternal can become an Ephemeral.
In the BBC Books spin-off novel The Quantum Archangel by Craig Hinton, it is mentioned that the Eternals co-exist with the Chronovores, Reapers, and the Guardians in Calabi-Yau Space, which is composed of six dimensions as opposed to our universe which is composed of five. It was also mentioned that Chronovores and Eternals were forbidden from mixing with each other, with this 'Covenant' being broken when a Chronovore and Eternal mate and conceive a child; although both parents are erased from history as punishment, their child survives to become Kronos, regarded as the greatest of the Chronovores, who is encountered by the Third Doctor when the Master seeks to control Kronos in The Time Monster.
The New Adventures range of Doctor Who novels published by Virgin Publishing featured the frequent appearance of an Eternal who called herself Death and seemed to be, in nearly every way, a personification of death itself. Some of the novels later stated that Death, along with Time and other Eternals, were worshiped as gods on ancient Gallifrey. The Doctor first met Death while in his seventh incarnation in the novel Timewyrm: Revelation. While battling the cosmically powered Timewyrm, the villain seemed to conjure up a representation of death that the Seventh Doctor then danced with. But this was not a creation of the Timewyrm's but rather the Eternal named Death who had been summoned by the Timewyrm's power.
Death taunted the Seventh Doctor in several books after that. In the novel Love and War by Paul Cornell, a dream sequence showed a conversation between the Doctor and Death and implied that the Doctor, while in his sixth incarnation, had actually planned his own death and rebirth into the seventh in order to become "Time's Champion." Death seemed to imply that she had been directly involved in this plan of action and that it may have resulted after a deal the Doctor made with her.
In the audio play Master, the Master is described as Death's Champion, just as the Seventh Doctor was said to be "Time's Champion." The audio play depicted how the Doctor made a deal with Death that she would make his arch-enemy the Master into a good man who did good things. Death agreed to this and gave the Master complete amnesia, after which she set him in a town called Perfugiam. He became a well-respected and loved physician named "John Smith" (coincidentally, an alias the Doctor himself has often used). But in return for turning the villain into a good man, Death said that the Doctor would have to kill him after ten years. The Doctor couldn't bring himself to commit this murder and soon afterwards Death restored the Master's memories, returning him to his villainous ways and making him hate the Doctor even more now for having stolen ten years of his life and "neutering" him.
In that same audio play, Death said that the Doctor had once killed a boy in self-defense when he was still just a child on Gallifrey. She said that she came to the Doctor afterwards in a dream and offered him a chance to be free of the guilt and memory of this crime, but that if he asked for it his best friend would be stricken with this guilt and memory instead. According to Death, the young boy who would become the Doctor agreed to this bargain and his friend believed he had done the killing instead. That friend eventually became the Master. Whether or not this account of Death's was true isn't known and can't really be proven, though the Seventh Doctor was very worried that he felt it was true and that perhaps he was responsible for beginning the Master's descent into darkness (although the Master's good side assured the Doctor that he didn't blame the Doctor for the actions he had committed as a child when he could not understand what was being offered).
In the novel So Vile a Sin, Death visits the Doctor when he suffers a heart attack - in only one heart - while attending the funeral of his companion Roz Forrester, telling him that she is coming for him directly very soon and that when he dies it will be alone, without companions and without meaning. This was meant to reference the Seventh Doctor's rather abrupt and lonely death in the 1996 Doctor Who television movie which had been released not too long before the book was published.
An article by Russell T Davies in the Doctor Who Annual 2006 stated that during the Last Great Time War between the Time Lords and the Daleks, the Eternals were so affected by the violence and the damage to space and time that they left the mainstream five-dimensional universe completely, never to be seen again.