The Chronovores were god-like transcendental beings who, as their name implied, fed on time itself. They were greatly feared, and sometimes respected or worshipped, because of this.
Nature
The Chronovores existed on another plane outside the universe, (PROSE: The Dark Path) usually in the Time Vortex or the Six-Fold-Realm, otherwise known as Calabi-Yau Space, (PROSE: The Quantum Archangel) the black void surrounding the universe where it was still expanding, (PROSE: No Future) or in the Morass. (COMIC: The Crimson Hand) As such, they were generally uncomfortable in the universe due to the lack of physical dimensions. (PROSE: Blood Heat)
They originally fed on the raw energy of stars (PROSE: The Dark Path) before evolving. After which they fed on time or aspects of time in various ways, essentially serving as the Third Law of Thermodynamics incarnate. (PROSE: Timewyrm: Genesys) They had three main sources of sustenance. In general, they could feed on the natural energy of the Calabi-Yau Space, but this was dull and unappealing food. Chronovores could also eat the history of a humanoid, which annihilated them from existence (they especially liked Time Lords), but their favourite food was alternate dimensions, which they tried to devour when they found them.
In addition to being related to the Eternals, (PROSE: The Quantum Archangel) they were also the "cousins" of the Temperon. (AUDIO: The Sirens of Time)
Appearance
Chronovores appeared as pure-white humanoids with small wings and helmet-like heads, but they could easily change their shapes like other transcendental beings, usually taking on feminie forms. (TV: The Time Monster) Artemis, described simply as a "bird-woman-thing", (PROSE: Conundrum) generally assumed the form of a woman in black, with black hair, red nails, and a red mouth of perfect teeth. She also wore a crown of thorns and diamonds, and manifested as a multi-dimensional mouth at one point. (PROSE: No Future)
Powers
In addition to their feasting on time and erasing people and things from history as a result, the Chronovores had great powers to shape reality, such as rescuing two TARDISes from a time ram. (TV: The Time Monster)
In her service to the Monk, Artemis changed events so that the Silurian Morka was able to kill the Third Doctor without the possibility of regeneration, thereby creating the Silurian Earth; (PROSE: Blood Heat) she again changed the Doctor's history so that the Garvond could come into existence; (PROSE: The Dimension Riders) she ensured Huitzilin's survival; (PROSE: The Left-Handed Hummingbird) she pulled a boy named Jason from Earth and installed him as the Master of the Land, after recreating the Land of Fiction from the ashes; (PROSE: Conundrum) and freed Varda from its time loop, enabling an invasion of 1976 Earth that never happened.
She later retroactively saved the Seventh Doctor from a fatal shot, made the Monk's mouth disappear and materialize a giant fishing hook through it, and changed history so that Ace saved Brigadier Lethbridge-Stewart's life. Because of this new timeline, Ace never set Artemis free, suggesting the Chronovores were able to "override" temporal paradoxes.
The blood of an Eternal was able to restrict them, suppressing their power. (PROSE: No Future)
Interaction with others
The Chronovores were feared and hated by other most other races, especially the Eternals who viewed them as parasites, (PROSE: The Quantum Archangel) and even the Third Doctor suggested Kronos to be a threat to the entire created universe, (TV: The Time Monster) the general consensus being that it was better not to attract the attention of the Chronovores to the normal universe.
There were, however, rituals used to summon them. One in particular involved a pentangle with the blood of five races whose genetic destinies had been altered by the Capitol on Gallifrey: Minyan, Silurian, Dalek, Human, and Mandrel. A Chronovore would consume the blood, making an image of the being appear and move around the pentangle as it grew older and died. Artemis commented that it was always the "same races."
Not being "normal life" or existing in the universe, concepts like "pain" and "obey" (PROSE: Blood Heat) were not understandable. Neither were the concepts of using a language or having a personality, and Artemis was content to eating Earth and every human as they had such short lives anyway before the Doctor convinced her she had a merciful personality. For freeing her, she even granted Ace a wish. (PROSE: No Future)
History
The ancestors of the Chronovores originally lived in the normal universe, able to move across time as other species moved across space. In the event that one of their kind grew tired or became ill and were stranded somewhere at some time, they built the Darkheart, a space/time conduit and medical device, to transmit energy to any point in space and time. They eventually evolved into a higher plane outside of time. (PROSE: The Dark Path)
They were led by the Divine Host, a group of particularly dangerous Chronovores and Kronos, a Chronovore/Eternal hybrid. (PROSE: The Quantum Archangel) The Chronovores were both feared and worshipped in Atlantis, which Kronos would later destroy during an encounter with the Third Doctor when the Master planned to enslave Kronos. (TV: The Time Monster) The Sixth Doctor later met the Master when Kronos took its revenge on him. (PROSE: The Quantum Archangel)
The Chronovores were present at the Timewyrm's creation, bestowing the name upon her. (PROSE: Timewyrm: Genesys)
The Monk later enslaved Artemis to get revenge on the Doctor, after which Artemis was freed and took her revenge on the Monk for imprisoning her. (PROSE: No Future)