Dark angel
From Tardis Wiki, the free Doctor Who reference
The dark angel was a creature engineered for use in an alien war.
It had limited chameleonic abilities, and was designed to infiltrate and destroy an enemy. It could sample genetic material to determine what targets to kill.
One of them missed its target and went into space. Its containment casket crashed on Earth and was brought to Kiev, where it rested under the city for centuries. Accidentally activated in 1240, it became known as the dark angel as it killed residents of the city. The First Doctor found its controlling device and sent information to fool it into believing its assassination mission was completed. It returned to its casket and self-destructed. (PROSE: Bunker Soldiers)
