The string

It wanted blood. Most of the time it was black as night, which helped when it moved around. It slinked into corners, dripped down passages, or when it needed to fly away in the night air. It plotted the death of its victims. Tripping old folks down staircases, unknotting itself at critical times, or just garroting the sleeping. It killed and destroyed, relishing the moment when it could drip in blood.