The mind scythe was Sontaran technology used to link two minds together to extract information.
Humans could be reduced to husks by being questioned with the device, although Time Lords like the Twelfth Doctor could survive the process and turn the effect against the Sontaran using the device.
Kygon Brox tried using the mind scythe to find out what The Doctor knew about the Warsong. Instead, The Doctor showed Kygon”[h]oliday snaps” of his recent adventures. As the link went both ways, The Doctor reversed it, and questioned Kygon about what the Warsong was. After doing so, The Doctor safely removed the connection with his sonic screwdriver. (The Instruments of War)