Anthony Stewart Head / Various
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The Krillitanes were an alien race who debuted in School Reunion. They had taken over the Deffry Vale comprehensive school on present day Earth, increasing the intelligence of the pupils with Krillitane oil. Using the children as part of a giant computer programme, they hoped to crack the secrets of the Skasis Paradigm, the Universal Theory that would give them control over the basic forces of the universe and turn them into gods. Their scheme was foiled by the Doctor and his companions, though not before they attempted to ask The Doctor to join them in remaking the universe. This ruse failed as miserably as the main plot of the Krillitanes.
The Krillitanes looked like a composite race who pick and choose physical traits they find useful from the species they conquer, incorporating them into their own bodies. When The Doctor last encountered them they looked like humans with very long necks, but by the time of School Reunion, they possessed a bat-like form which they obtained from the conquest of Bessan ten generations prior. However, they were able to maintain a morphic illusion of human form, which could be discarded if needed.