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.

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; the trap street in London, where 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. The mechanism of the chronolock allowed for it to be transferred only once to someone willing to take it on; that person would then die in the place of the original condemned. Mayor Me had the authority to remove the chronolock from the original condemned, though usually chose not to in order to maintain rule of law, given the fact her camp included many warlike species (including Cybermen and Sontarans) that would not respect her authority if she rescinded punishment. Me could not remove the chronolock from after it had been passed on. The Quantum Shade was physically connected to Me by way of a tattoo-like marking on her chest that would trigger the raven's attack on the condemned.

The Quantum Shade rendered a quick, but painful death on a target. At one point, Me says the Shade is "promised a soul" but it's unclear whether this is literal.

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 (even though doing so rendered it impossible for him to say goodbye to his family). After discovering the tattoo and 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 Rigsy, 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. She managed to convince Rigsy to give her the chronolock.

However, they soon discovered the "murder" was staged to be a trap for the Doctor and Clara. Me had made a deal with the Quantum Shade that she could remove the chronolock from Rigsy at any time. Attempting to do so, Me and the Doctor discovered that Clara had transferred the chronolock to herself.

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 (notably, when the raven was triggered, the markings sported by Me did not change).

After saying goodbye to the Doctor, Clara was killed by the Shade as the horrified Doctor watched. (TV: Face the Raven)

This event led the Doctor into a desperate, 4.5-billion-year bid to somehow save her life. Returning to the moment before the raven entered her body, the Doctor withdrew Clara from the timestream, though ultimately he was only able to delay the inevitable. The chronolock marking remained on Clara's neck and although she had untold adventures on her own and travelling with Me, eventually the laws of time demanded that she return to the moment of her death, allowing the Quantum Shade's contract to reach its conclusion. (TV: Heaven Sent / Hell Bent)

Behind the scenes
Sarah Dollard, writer of Face the Raven, has said on her Twitter that once the Chronolock is transferred to another person and Ashildr is cut from the deal, the Quantum Shade can't be stopped by anyone.