Krillitanes
The Krillitanes first appeared in School Reunion (Series 2 : Episode 3) as large, bat like creatures intend on breaking the Skasis Paradigm to become Gods. The Krillitanes, from the planet Krillia, are a composite race, who pick and chose body parts and traits that they find useful from species that they’ve conquered and incorporating them into their own bodies. It is said that when the Doctor last met them, before School Reunion, that they appeared human with very long necks. When we see them in School Reunion, they possess a bat-like form, with large wings, though they can still maintain a morphic illusion of a human form. They obtained this bat-like form after the conquest of Bessan ten generations prior. The Krillitanes are carnivorous, and eat many sentient creatures, including rats.
The Krillitanes produced Krillitane oil, which was used to boost the intelligence of the pupils at Deffry Vale comprehensive school. The Krillitanes had infiltrated the school as teachers, and were using the children’s intelligence and imaginations to help them break the Skasis Paradigm. This Universal Theory would give them the ability to become Gods, and they even tried to tempt the Tenth Doctor into helping them. Through the Krillitanes rapid evolution, their own oil had become toxic to them, reacting in a similar way to acid. This oil was used to stop the Krillitanes when K9 assisted the Doctor in trapping the Krillitanes and exploding the oil, at the cost of K9’s life.