The Krillitane are a race of Aliens that appeared in 2006 in the episode School Reunion, starring Anthony Head (Merlin) as the leader of the alien race on present day Earth. They had taken over Dreffy Vale Comprehensive School by killing the teacher and then wearing their skin, using Krillitane Oil to massively increase the intelligence of the pupils at the school. Using the pupils as part of the giant computer programme, the Krillitane hoped to open and reveal the secrets of a high-Tech device known as the Skasis Paradigm, it is meant to contain the universal theory that would give the Krillitane control over the basic forces and hence rendering them 'God-Like' to the Human race. However this scheme was soon discovered and foiled by the Tenth Doctor (Played by David Tennant) and his companions, although, in a last attempt asking the doctor to join their forces and help them in remaking the universe. This gives hint as to the Krillitanes awareness of the Time War and try to tempt the doctor into rebuilding his race and henceforth, breaking the time lock on the last war.
The Krillitane are a race that pick and choose life forms that they can compose the form of for thier desired needs. They are bat like in appearence and like the test of Human flesh, but only because they desire their skins. It is said theat they obtained their 'bat-Like' look from the conquest of Bessan, ten generations prior to the revision of the race. The last time that the doctor saw them, they looked like humans with long necks.
However, the side-effects of their rapid evolution made the same oil they are using to up the intelligence of the pupils of Dreffy Vale.