Ouroboros is the term used on the case revolving around the mass serial killings that originally ran from 2009-2018.
Ouroboros refers to the "snake that eats its own tail", similar to how this killer leaves his victims on a circular bus route. In the past, the killer successfully murdered and publicly displayed his 38 victims, all of whom were child molesters, or those who kept their secrets. The killer believed himself to be an avenging vigilante. His modus operandi consists of disfigurement, particularly the burning out and removal of their eyes while under decamethonium, and suffocation by forcing doll heads, laced with melamine, down their throates. The victims are then left in similar poses, all looking towards the road.
Since Kiera Cameron was from the future and was able to study the case in the future, she was able to catch the Ouroboros killer, later revealed to be a pair of doctors: David Eldridge and Michael Venables, Dave's patient and later employee. Dave was dying of cancer, and his apprentice, Mike, planned to continue his plan of killing the 30 other people on Dave's list. Solving the case and killing Mike by 2013 prevented him from proceeding with the killings.
In the future, though, another killer seems to have taken up the Ouroboros' killer's cause, or is at least committing murders with just a similar profile. By the time Kiera is brought into the case, the new killer has been killing for three years and has just completed his 13th murder.