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 . Unable to shape - shift , they could create morphic illusion s , keeping them cloaked in human form . Their wing s were capable of lifting heavy things . When the Doctor first encountered them , the Krillitanes looked like human s 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 because of continued overuse and physiological alterations . ( 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 bat s and had very sensitive hearing . The Krillitanes were carnivorous and would eat other sentient creatures , but they would also eat rat s . ( TV : " School Reunion " ) Krillitanes also produced Krillitane oil . ( TV : " School Reunion " , PROSE : " The Krillitane Storm " ) As an unfortunate side - effect from their sensitive ears , 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 when cloaked as a human . ( TV : " School Reunion " ) Technology . Krillitanes had spaceships and were capable of creating Deadlock seal s . ( 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 Lord s , Brother Lassar regarding them as " a pompous race " and " ancient , dusty senators " who feared change and chaos . ( TV : " School Reunion " ) History . Ten generations before 2007 , the Krillitane conquered Bessan , where they killed one million of its species on one day . 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 " ) In 2010 , Krillitanes tried again to improve humans ' brains with their oil employed in the Brainy Crisps ; without knowing , the enhanced consumers of crisps helped design a new form for their species . Eventually the plan was foiled by the Tenth Doctor and Henry . ( PROSE : " Code of the Krillitanes " ) Other references . When Sarah Jane Smith and Maria Jackson asked Mr Smith to identify the species of Uvlavad Kudlak , Mr Smith briefly showed them an image of a Krillitane . ( TV : " Warriors of Kudlak " )