The Enemy (Captain Jack Harkness)

Jack Harkness recalled that, when he was a boy, he went to war alongside his best friend against an enemy whom he described as "the worst possible creatures you can imagine". Jack had persuaded his friend to join up, saying that it would be an adventure. However, they got caught crossing the border over enemy lines. The enemy chose to torture Jack's friend because he was weaker than Jack. Ultimately, they let Jack go, but not before making him watch his friend die. Jack observed that his friend "hadn't lived." (TV: Captain Jack Harkness)

Jack had came from the Boeshane Peninsula in the 51st century, living under the threat of invasion. Ultimately, what Jack described as "the most horrible creatures you could possibly imagine", whose howls travelled before them, arrived. They had been known to pass over the Peninsula and it was believed that they would do so again, but it was on that day that they attacked. Jack's father, Franklin, had sent him away with his younger brother Gray to run for safety as he went to find their mother. As they ran, however, Jack lost Gray after losing the grip on his hand. Jack returned home to find his mother with Franklin's corpse. (TV: Adam)

Gray himself was captured by the creatures who, as he recalled, "lived to torture". They kept him and others just on the verge of life and he would lay, hemmed in by corpses, praying to become one. Years later, the now adult Gray sought retribution against his brother following these experiences. (TV: Exit Wounds)