Chronolock

A chronolock was a force which bound Quantum Shades to their intended victims.

Function
Chronolocks appeared as a tattoo on the back of the victim's neck that counted down the minutes until their death. They could be passed from one person to another by mutual consent. Once a chronolock appeared, the death was considered "locked in" - it could be passed from the victim to another person, but not avoided entirely.

An exception to this rule existed, however; if the chronolock was applied by an individual who had allied themselves with a Quantum Shade, it could be removed by that same individual. This only worked if the chronolock remained with the intended victim; if it was transferred to somebody else, or in any way altered by an outside force, the agreement made between the Quantum Shade and its ally would be invalidated, and they would no longer have the power to cancel the chronolock.

Quantum Shades could follow chronolocks anywhere, over any geographic or temporal distance; the Twelfth Doctor believed that they were impossible to remove or conceal, other than through the aforementioned method. The exact functioning of chronolocks, however - how Quantum Shades are able to track them, and how they are applied to their victims - remains unclear. (TV: Face the Raven)

Though ordinary time travel would not prevent a chronolock from counting down, or deter a Quantum Shade from reaching its target, an individual whose physical processes had been time looped using Gallifreyan technology could avoid the ramifications of the chronolock for a theoretically infinite period of time. (TV: Hell Bent)

History
After having been marked by a Kotturuh in the Dark Times, the Tenth Doctor scanned himself for a chronolock only to conclude that the Kotturuh didn't use them. (PROSE: The Knight, The Fool and The Dead)

Sometime during or prior to the early 21st century, the immortal human Ashildr, going by the name Mayor Me formed an alliance with a Quantum Shade which took the form of a raven, as part of her attempts to protect the Trap Street, of which she was mayor. Ashildr would use the Quantum Shade as a deterrent, applying a chronolock to those who broke the rules of the street. When she sentenced Rigsy to death for the supposed murder of Anah, a chronolock appeared on his neck.

After Rigsy returned to the trap street with the the Doctor and Clara Oswald, they witnessed a man - whom Ashildr had sentenced to death for stealing medical supplies from the street - refusing to pass his chronolock on to his wife; shortly after, his chronolock reached zero, and the raven took his life.

Clara convinced Rigsy to pass his chronolock to her, to ensure his safety. However, Clara failed to realise that doing so changed the terms of the agreement that Ashildr had made with the Quantum Shade; as a result, the chronolock could no longer be removed, and Clara was killed by the Quantum Shade. (TV: Face the Raven)