Neural block

A neural block was a small Time Lord device that wiped memories. The Twelfth Doctor tried to use a human-compatible neural block on Clara Oswald to wipe Clara's memory of him, but Clara reversed the polarity using the sonic sunglasses and instead, the Doctor forgot most of his memories of Clara. (TV: Hell Bent) They were eventually restored by a glass avatar of Clara from the Testimony. (TV: Twice Upon a Time)