The Enemy

The enemy was the opponent of the Time Lords during the War. (PROSE: Alien Bodies) It was not a species or a political faction as much as a process. It had a name, but the Great Houses were reluctant to use it. (PROSE: The Book of the War)

Identity
Before the War, multiple parties speculated about the enemy's identity. Chatelaine Thessalia incorrectly predicted in The Little Book of Absolute Power that the enemy would be mainly motivated by survival or keeping its history intact. (PROSE: The Book of the War) Greyjan the Sane hypothesized that the Enemy were ancestor cells which had been irradiated by temporal interference and energised by a leaking bottle universe. (PROSE: The Ancestor Cell) The Eighth Doctor offered the more entertaining possibility that they were "Yartek, leader of the alien Voord, carrying a big stick." (PROSE: The Taking of Planet 5) The Book of the War speculated that the increasing paranoia on the Homeworld may have caused the War, saying the idea "makes a certain sense, given the nature of the enemy." (PROSE: The Book of the War)

Some legends indicated that the Enemy was descended from a Type 101 timeship who was so indistinguishable from a human that it lived and died as one. (PROSE: Of the City of the Saved...) Compassion was adamant that the enemy originated from Earth, listing it as one of the reasons that the Time Lords would want to destroy the planet in the 20th century. (PROSE: Interference - Book Two)

In her speech to the Fifth Wave on the thirtieth anniversary of the Cataclysm, House Military strategist Entarodora said the ruling Houses kept secret the identity of the enemy and its leadership because, if the Houses believed the true enemy was simply a rogue House or a species of time-active upstarts, they would "simply shrug and go back to sleep." Instead, the secrecy made the enemy into monsters, and the subsequent fear would keep the fight alive.

The Book of the War posited that the name was kept secret because the "why" of the enemy was more important than the "what". It also speculated that the enemy might come from beyond the Great Houses' noosphere, not because it came from another universe but because it operated on principles that the Houses weren't built to understand; it said that, unlike the Yssgaroth, the enemy was "civilised, cultured, and intelligent enough to have an agenda beyond pure destruction."

According to Carmen Yeh's heavily-fictionalised memoir Fantastical Travels in an Infinite Universe, Compassion believed that the enemy was a meaningless distraction and the real threat to the Homeworld would come from within, specifically "House Lucia" or "family". (PROSE: The Book of the War) The Egyptian god Horus described Lolita as a "process" and "a new kind of history". (AUDIO: The Judgment of Sutekh) The Book of the War similarly described the enemy as a process and the War as "a struggle between one kind of history and another." (PROSE: The Book of the War)

When asked who could fight a war against a race of gods, Abschrift quickly replied, "Who indeed." (PROSE: Warlords of Utopia) The Doctor said he wasn't working for the High Council and was only one of the enemy "depend[ing on] where you're standing." (PROSE: Alien Bodies)

Powers
The enemy was seemingly bound by the same Protocols of Linearity as the Houses: it was unable to attack vulnerable points in the Homeworld's history, instead encountering the Houses in the same order that it was encountered by the Houses. This was supported by the initial battle on Dronid, in which the enemy's forces were as ill-prepared as the Houses' First Wave.

Many Great House academicians posited that the enemy was farther along in their ability to use high chaotic limiter settings.

The Book of the War said the enemy "tries not to" violate the laws of physics. (PROSE: The Book of the War)

Qixotl noted that the enemy could wipe out information just as fast as they could destroy matter. (PROSE: Alien Bodies) In its entry for the enemy, The Book of the War referenced its articles on the Churchill Index, Immaculata Formosii, the Gods of the Ainu, "Miss Hiroshima", Mohandassa, Sixth Wave Defections, S'tanim, and Violent Unknown Events; however, none of these entries existed in the book, implying that the enemy had tampered with the text. (PROSE: The Book of the War)

Representatives
House Military soldiers used the colloquialism "Rep", an abbreviation of "representative", to refer to the enemy's agents. (PROSE: The Taking of Planet 5) Tonton Macoute once cooked the corpse of an enemy soldier. (PROSE: Tonton Macoute)

The enemy had a small automated outpost on Simia KK98; there, a Gabrielidean soldier working for the Time Lords was wounded by a combat satellite. As died, the Doctor visited him; later, the soldier was rescued and turned into a Shift by people dressed in flowing robes with high collars, designed as parodies of the costumes of the High Council. These agents sent him, now calling himself Mr Shift, to represent the enemy at Qixotl's auction for the Relic. (PROSE: Alien Bodies)

In the Mount Usu duel during the filming of Mujun: The Ghost Kingdom, Chris Cwej and Michael Brookhaven encountered an enemy agent which apparently summarised its own mystique: "The Scourge. Harvey. Hermes. The coolest character is the one whose face we never get to see." It was represented by the total absence of anything on the recovered film, appearing not as blackness but instead as emptiness and background filmstock. (PROSE: The Book of the War)

On Roma CLII, Marcus Americanius Scriptor killed a creature that was hunting a renegade who had escaped the War by jumping into a parallel universe. The monster was implied to be working for the enemy. (PROSE: Warlords of Utopia)

Parallel universe
In a parallel universe where the Second Doctor was pardoned instead of being exiled to Earth, the Sixth Doctor was Lord President Admiral of Gallifrey during the War. There, the Enemy had always had access to rudimentary time corridors and travel machines, but they gained temporal manipulation powers after the Master defected to their side. The Doctor described them as an old foe with calculating tin minds and jet-black saucers. (PROSE: The Quantum Archangel)

Behind the scenes

 * When they were first mentioned in Alien Bodies, the Time Lords' foe in the war was called just "the enemy", without any capitalisation; this format was followed in The Book of the War and the rest of the Faction Paradox series. However, following on a brief uppercase reference to "the Enemy" in Unnatural History, the proper-noun title was employed in later BBC Books novels The Taking of Planet 5, The Quantum Archangel, and The Ancestor Cell.
 * Though Lawrence Miles originally intended the enemy to be an "unseen-and-unknowable factor", by the time he finished writing Alien Bodies, he'd "figured out exactly what was going on and why". However, "moods changed", and he came up with a whole list of other possibilities. None of them were good enough to be the definitive answer, but eight were almost good enough, one of which was a "whale-king" fashioned after the of European superstition. Miles intended to reveal the enemy's identity in his final Doctor Who novel, and he asked the BBC if they could publish a book where one single page had eight different versions, so the revealed enemy would depend on which book a person purchased. Range-editor Stephen Cole shot down the idea because he preferred to keep the enemy's identity a complete mystery.