From TARDIS Index File, the free Doctor Who reference.

Krillitane
Appearances:	DW: School Reunion, NSA: The Krillitane Storm, Code of the Krillitanes, IDW: Fugitive
Notable Individuals:	Lassar

The Krillitanes were a race of aliens who could take physical features from races they conquered.

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.
• In 2007, the Krillitanes were bat-like humanoids, though smaller than Humans. They had various shades of skin ranging from pinkish to charcoal gray, sharp claws and a flattened, fanged face. They took their bat-like wings from the inhabitant of Bessan, and they also 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. The Krillitane oil had become toxic to them.
• In 2007, Lassar became completely Human, but shared the original Krillitane's carnivorous habits.

History

10 generations before 2007, the Krillitane conquered Bessan, killing millions. They took bat-like wings from the inhabitants.

In 2007, a group of Krillitanes led by Brother Lassar landed in Deffry Vale 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. The Doctor, Rose, Sarah Jane and Mickey were able to stop them. (DW: School Reunion)

Lassar was able to survive the defeat, but was trapped in Human form. When the Doctor was put on trial by the Shadow Proclamation, Lassar served as prosecutor. (IDW: Fugitive)