The Cloister Wraiths, colloquially known as Sliders, were creatures found in the Cloisters, a crypt beneath the Capitol of Gallifrey which provided access to the Matrix.
The Wraiths were apparitions of dead Time Lords whose minds had been uploaded to the Matrix, designed to guard it much like a firewall. When the Twelfth Doctor returned to Gallifrey, the Wraiths became very active, ringing all of the Cloister Bells as a warning to the Time Lords. Entering the Cloisters, the Doctor and Clara Oswald discovered various creatures including Daleks, Cybermen and Weeping Angels who had entered the Cloisters determined to steal the Matrix's database secrets, all of which had been attacked and "filed" by the Wraiths, trapped forever. Only one individual, a student of the Time Lord Academy, was rumoured to have entered the Cloisters and escaped the Wraiths, by finding a secret passage with their help: this student was the Doctor. (TV: Hell Bent)