Quantum Shade

The quantum shade was a being that killed specific people its master marked. According to The Doctor, it would find a target marked for assassination no matter where in time or space they fled. It took the form of a raven.

The mark on the target looked like a tattoo on their neck, counting down the minutes to their death.

Involvement

At some point, Me allied herself with the quantum shade. She then built an alien refugee camp in London, using the quantum shade to enforce the law. Whoever broke the law would be marked for assassination, but given enough time to say goodbye to their loved ones.

When Rigsy was accused of murdering one of the refugees, he was marked for death and retconned. After calling the Doctor and Clara for help, they found the camp and started to look for 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.

However, after they figured out the "murder" was staged to be a trap for the Doctor and Clara revealed to have taken the mark, Me told them that if someone took the mark from a target, she was unable to remove it and the new target was inevitably doomed. Clara was eventually killed by the quantum shade. (TV: Face the Raven)