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. They could also create morphic illusions, giving them the appearance of yet other species.

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

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. (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, but they would also eat rats. (TV: School Reunion)

In 2007, Brother Lassar became completely human. (TV: School Reunion)

Krillitanes also produced Krillitane oil. (TV: School Reunion, PROSE: The Krillitane Storm) 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. (TV: School Reunion)

Technology
Krillitanes had spaceships and were capable of creating Deadlock seals. (TV: School Reunion)

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 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." (TV: School Reunion)

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

History
Ten generations before 2007, the Krillitane conquered Bessan, killing millions. They took bat-like wings from the inhabitants. (TV: School Reunion)

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 as a type of computer in order to work out the Skasis Paradigm. 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)

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 Micro-Universe Series.