Chronolock

A chronolock was the force that bound the Quantum Shade to its intended victims. It appeared as a tattoo on the back of the victim's neck that counted down the minutes until their death. The chronolock could be passed from one person to another by mutual consent.

When Mayor Me sentenced Rigsy to death for supposedly murdering Anah, a chronolock appeared on his neck.

After Rigsy returned to the trap street with the Twelfth Doctor and Clara Oswald, they witnessed the Quantum Shade kill a man when his chronolock expired.

Clara convinced Rigsy to pass his chronolock to her to protect him. However, doing so changed the terms of the agreement that Me originally made with the Quantum Shade, such as that Me could no longer remove the chronolock. As a result, Clara was killed by the Quantum Shade. (TV: Face the Raven)