Krillitane

The Krillitanes were a race that could take physical features from races they conquered. They would "cherry-pick" the best bits from the people they destroyed.

Biology
The Krillitanes were able to take the abilities from other species, giving them new appearances and qualities. Unable to shape-shift, they could create morphic illusions, keeping them cloaked in human form. Their wings were capable of lifting heavy things.

When the Doctor first encountered them, the Krillitanes looked like humans with very long necks. (TV: School Reunion)

Historians who had been allowed access to the "Monster Vaults" of the databanks in the Doctor's TARDIS likened the Krillitanes' acquiring physical abilities from other species to the Abzorbaloffs of Clom, who absorbed the thoughts and experiences of other lifeforms. Additionally, the Abzorbaloffs were identified as using a type of morphic illusion to disguise themselves like the Krillitanes. (PROSE: The Monster Vault)

In 1140, the Krillitanes were in the initial stages of their bat-like forms, having only recently conquered Bessan and, as such, had hooked tails and longer necks. This gave them the impression of the devil to the human natives of Earth at the time. The Krillitane oil had become toxic to them by this point because of continued overuse and physiological alterations. (PROSE: The Krillitane Storm)

In 2007, the Krillitanes had become more human-like, with shorter necks and smaller tails. They had various shades of skin ranging from pinkish to charcoal grey, sharp claws and a flattened, fanged face. They slept upside down like bats and had very sensitive hearing. The Krillitanes were carnivorous and would eat other sentient creatures, such as humans, but they would also eat rats. (TV: School Reunion)

Krillitanes also produced Krillitane oil. (TV: School Reunion, PROSE: The Krillitane Storm) As an unfortunate side-effect from their sensitive ears, they were highly susceptible to high-pitched noises, such as a fire alarm. They were also very strong, as Lassar was able to punch through a wall and rip out the power cable to the fire alarm when cloaked as a human. (TV: School Reunion)

Technology
Krillitanes had spaceships and were capable of creating deadlock seals. (TV: School Reunion) As the Tenth Doctor observed, they seemed to change the design of battlecruisers every time they amalgamated a new race. (COMIC: Fugitive)

Culture
Krillitanes apparently referred to each other in familial terms. Toeclaw was known as Brood-Mother, (PROSE: The Krillitane Storm) while Lassar was known as Brother Lassar. (TV: School Reunion) The Krillitane served under a Brood-Mother. (PROSE: Father of the Daleks)

The Tenth Doctor reacted in fear when he discovered the oil used on Deffry Vale High School's chips was Krillitane oil. When Rose asked if the Krillitanes were bad, the Doctor responded, "Think how bad things could possibly be and add another suitcase full of bad."

They knew of the Time Lords, Brother Lassar regarding them as "a pompous race" and "ancient, dusty senators" who feared change and chaos. (TV: School Reunion)

History
The Tenth Doctor disclosed whilst traveling with Rose Tyler that he had earlier encountered Krillitanes who appeared as humanoids distinguished by their "really long necks". (TV: School Reunion)

Ten generations before 2007, the Krillitane conquered Bessan, where they killed one million of its species on one day. They took bat-like wings from the inhabitants. (TV: School Reunion) The Esteemed Father, himself of the older generation of Krillitanes who lacked wings, was recognised as the "Beast of Bessan".

In 1140, Lozla Nataniel Henk attempted to breed Krillitanes in Worcester, but was defeated by the Tenth Doctor. By using their own oil against them, Henk attempted to create a huge, mutated Super Krillitane that he could sell to the highest bidder as a weapon. However, the creature was killed when Captain Darke threw a vial of Krillitane oil into its mouth as it was rampaging around the city of Worcester. (PROSE: The Krillitane Storm)

In 2007, a group of 13 Krillitanes led by Brother Lassar landed in Deffry Vale High School in London. They took over Deffry Vale High School, disguising themselves as the teachers. Using Krillitane oil and an advanced computer network, they were using the brains and imagination of the students, enhanced by Krillitane oil, as a type of computer in order to work out the Skasis Paradigm (TV: School Reunion) to control time and space. (COMIC: Fugitive) Mickey Smith deactivated the computer network and evacuated the students the Krillitanes were using, while K9 Mark III sacrificed himself, killing the Krillitanes when he blew up a vat of Krillitane oil with his laser. (TV: School Reunion)

In 2010, Krillitanes tried again to improve humans' brains with their oil employed in the Brainy Crisps; without knowing, the enhanced consumers of crisps helped design a new form for their species. Eventually the plan was foiled by the Tenth Doctor and Henry. (PROSE: Code of the Krillitanes)

A year before defeating the Auctioneers, Kate Stewart and Petronella Osgood found a Krillitane detonator in a power station. (AUDIO: Game Theory)

By the 52nd century, a Krillitane Empire existed. Rumours spread of the Empire planning to begin a war on other species. After learning Dorium Maldovar had left in a hurry, Krillitane spy Agrakos led a raid on the Maldovarium to obtain 70 DNA samples of rare species to enhance the Krillitanes in preparation for the war but was foiled by Dorium's Ood. (PROSE: The Heist)

Shortly before the Last Great Time War, a group of Krillitane rebels based themselves at a facility in Gryphon's Reach to develop a phase-cloak that would allow them to perfectly hide from their species, being aided by the Eleventh Doctor. When these experiments drew the attention of the Dalek Empire, the Doctor combined Gallifreyan and Krillitane bio-technology to cloak the Daleks' sensors and allow the rebels to escape. (PROSE: Father of the Daleks)

Other references
When Sarah Jane Smith and Maria Jackson asked Mr Smith to identify the species of Uvlavad Kudlak, Mr Smith briefly showed them an image of a Krillitane. (TV: Warriors of Kudlak)

When Mickey Smith and Martha Jones were investigating the gargoyles plaguing San Francisco in 2016, they initially mistook the creatures for Krillitanes. (COMIC: The Transformed)

Behind the scenes

 * Krillitanes were originally called Krillians, until a check revealed that this was the name of a real computer company. (DWDVDF 7)
 * Russell T Davies' original concept for the Shadow Proclamation scene in The Stolen Earth included an appearance by several swooping Krillitanes, but his plans to feature a "rogues gallery" of alien races in the scene fell through; ultimately the Judoon were the only returning race featured.
 * Krillitanes are included in the Character Options Micro Universe action figure series.
 * On the Series 2 outtakes reel, there are "bloopers" featuring animated creatures from the series getting into antics, such as the Krillitanes and the werewolf, done just for fun.
 * The Krillitane bore strong resemblance to the Malevilus from the comic story Doctor Who and the Iron Legion.