Sleeper agent

Imperial Intelligence often used torture and various methods of brainwashing to create sleeper agents: agents who were unconsciously conditioned to perform some task. Their "patients" would be allowed to reunite with their targets, for example returning to the Rebellion, after conditioning; they themselves would have no memory of what had happened to them or what they were conditioned to do. Then, at an opportune time, Imperial Intelligence would activate the agent's conditioning, causing them to commit murder, sabotage, theft, or other undermining acts without the ability to control themselves.

Text and facts from the “Sleeper agent/Legends” article on Wookieepedia, licensed under CC BY-SA 3.0. Modified for this site.