Eye Drive

The Eye Drive was a device created by the Silence to allow selected individuals, such as Madame Kovarian, to remember the memory-proof alien species running the order. The Eye Drive looked like a black eye patch and was worn over a person's eye. The device formed a neural link with the user's mind and continually reminded them of the existence of the Silents.

The Silence built a fail-safe into the devices that caused pain or death when activated, depending on the individual. This fail-safe was integral to the functioning of the device; any duplicates included it. (DW: The Wedding of River Song)