Quantum Shade

Quantum shades were beings that could kill specific people regardless of their location in space and time. Their victims would be marked with a chronolock, a tattoo-like brand on their neck, counting down the minutes to their death.

History
At some point, Me allied herself with a quantum shade that had taken the form of a raven, using it to enforce her law in an alien refugee camp in trap street in London over which she was mayor. Whoever broke the law would be marked with a chronolock, given enough time to say goodbye to their loved ones before their death.

After Rigsy was lured into the camp and accused of murdering one of the refugees, he was marked for death by the shade and Retconned prior to his release. After calling the Twelfth Doctor and Clara Oswald for help, the trio located the camp and investigated the crime in search of proof of Rigsy's innocence. When Clara learned that someone else could take the mark, she volunteered to take it from him, not wanting his daughter to grow up without a father and believing Me would save her since she had declared Clara to be under her protection, and managed to convince him to give her the chronolock.

However, after they discovered the "murder" was staged to be a trap for the Doctor and Clara revealed to have taken the mark, Me informed Clara that, in taking the chronolock, she had removed Me from any deal she had made with the shade, rendering her unable to remove it as she normally could. To the Doctor's dismay, Clara was killed by the shade. (TV: Face the Raven)

Other matters

 * Another type of Shade, an Elemental, killed Elton Pope's mother in the 1970s. It had escaped from the Howling Halls. (TV: Love & Monsters)