A cortex leech was a parasitic creature that killed hosts before taking control of their corpses. Whilst inhabiting a corpse, it drained energy from those nearby to sustain itself, without which the body would liquefy.
A number of leeches arrived in Cardiff through the Rift and were made known to Torchwood Three. They targeted the homeless and caused a significant amount of deaths. Toshiko Sato studied them and wrote a report which she hurried for Suzie Costello. Tosh and Sebastian Vaughan attempted to collect a leech unsuccessfully a number of times.
When Tosh and Sebastian found one, Sebastian was killed as Tosh had neglected to bring the equipment necessary to stop it, having been rushed by her impatient partner. Sebastian was killed and later reanimated by the leech at Rockfell Hall, where the leech attempted to drain Madeline Vaughan's energy. Tosh stopped it and Sebastian's body melted. (AUDIO: The Vigil)