- You may be looking for Elemental Shade.
Quantum Shades were beings that could kill specific people regardless of their location in space and time. They seem to desire it, with Me explaining that she "promised" her Quantum Shade ally "a soul". Their victims would be marked with a chronolock, a tattoo-like brand on their neck, counting down the minutes to their death. They could appear as billowing clouds of smoke or "compact" themselves into innocuous physical forms. Deaths caused by Quantum Shades were automatically fixed.
As Mayor of the Trap Street, Me made a contract with a Quantum Shade known to the Street's residents as "the Raven", as it often took the form of a large black raven except when it went "hunting" for the victims Me offered it. She used the threat of the Raven to keep a ruthless order in the Street, allowing it to function as an alien refugee camp that would accept even the likes of Sontarans and Cybermen. Though Me had not intended it, Clara Oswald eventually became a victim of the Raven, (TV: Face the Raven) although the Twelfth Doctor later managed to extract her from her time-stream just before death, allowing her to live out an arbitrarily long life without aging until she eventually chose to be returned to her appointed death. Although Clara did seem to intend to fulfill this obligation eventually, (TV: Hell Bent) the Doctor later came to suspect that, given this infinite time, she found a way to bypass this death entirely, although she eventually died in unrelated circumstances, with her mind later becoming part of the Testimony. (PROSE: Twice Upon a Time)