The Control Shackle is a magical collar-like device that allows the user to forcefully control someone.
The Control Shackle manifests as a green ring around the target's throat. If they do not obey, they will feel an electric shock until they do. When under the user's control, the target's eyes will glow bright green.
In Source Material, the shackle was used by Fiona Callahan to control Harry Greenwood. She would order him to do something, and if he refused she would make the shackle choke him and also send an electric shock. She would also use it to torture him should he do something she didn’t want him to.