He is an exorcist, demonologist and master of the dark arts. Raised by an abusive father who blamed him for the death of his mother in childbirth, John grew up highly cynical and cunning, and developed an interest in the dark arts. In his teenage years, he was introduced into the occult world by Anne Marie, learned various spells and became an exorcist with the hopes of one day using his powers to resurrect his mother. John became a figure of respect among the dabblers of magic.
However, one night, John attempted an exorcism on a young girl named Astra by summoning the demon Nergal, only for the demon to kill Astra and drag her soul to Hell, damning her for eternity, an action which damned John's soul for Hell upon death.
Overwhelmed with guilt John initially retired but after hearing of a Rising Darkness from the angel Manny, he returned to face this threat with Manny giving him hope that should he succeed his soul may be spared eternal damnation.
John was born in Liverpool in 1982, though his mother died giving birth to him. Her death caused his father to develop a deep resentment towards John, whom he blamed for Mary Anne's demise, and led to him calling his son "Killer" throughout his whole life. Additionally, whenever his father came home from drinking at the bar, he would hit John in the face and remind him that his mother died because of him. Also, to punish him, John's father would put out cigarettes by burning them on John's arms. John also had an older sister.
During John's teenage years, he met a nun named Anne Marie, who introduced him into the occult world. The two also became lovers. John then began reading every book he could find on the occult, learning how to conjure dead beings in the hopes of one day conjuring his mother.
At some point in his life, he met and became friends with both Chas Chandler and Esrin Fortuna, the latter of whom he eventually came to owe a substantial sum of money.
He also had a brutal encounter with sorcerer Felix Faust which he survived but left Faust with a grudge against him, and also heard of the voodoo priest Papa Midnite. He also heard of Damien Darhk, leader of H.I.V.E., and came to fear him so much he considered leaving whatever city he was seeking to destroy a preferable option to fighting him.
Visit to Lian Yu
Seeking an object called the Orb of Horus, an ancient spell book, John traveled to Lian Yu to retrieve it but was caught by Baron Reiter's men and interrogated, violently. Conklin interrupted the interrogation to tell Reiter of an A.R.G.U.S. communications device he found and accuses newcomer Oliver Queen of being a spy. Reiter presented Oliver with a map John possessed and after confirming his knowledge of their locations, John performed a spell to free himself from his handcuffs and held Oliver at gunpoint and used him as leverage to escape Reiter's compound. He had Oliver forcibly take him to the area he sought and performed a spell to uncover the hidden passageway, much to Oliver's shock. Once inside, they explored the cave where John explained that there are areas of the world as old as it including Lian Yu, possessing things corrupt men aren't supposed to have such as Reiter. John eventually found what he sought and retrieved it but set off a trap which Oliver saves him from being killed by, insisting he's not one of Reiter's men. After exiting the cave John thanked Oliver for saving his life and broke off the crystal off the rod to give to Reiter, as the rod itself is what contains the magic. John also warned Oliver about Reiter, insisting his narcotics aren't his only interest for being on the island. As insurance John also used the rod to transfer a spell on his arm onto Oliver's torso, insisted he'd know what to do when the time came and on Oliver's request, John punched him out cold as part of his "escape" and left on his boat, but the two would keep in contact.
Incident at Newcastle
A demon possessed a girl named Astra, the daughter of a friend of John's and he and his friends Anne Marie Flynn, Gary Lester, Ritchie Simpson, Judith and Chas Chandler attempted to stop it. John, however, decided to summon a more powerful demon named Nergal and extort him into dragging the lesser demon back to Hell, but unfortunately Nergal was able to seize Astra and killed her violently in front of everyone then pulled her soul into Hell damning her for eternity. This action severally traumatized John and he placed himself in Ravenscar Psychiatric Facility for the Mentally Deranged out of guilt. The rest of the crew also came to resent John, some more than others, but regardless his actions and Astra's death haunted them all for the rest of their lives.
Paying an old debt
A few months after his voluntary interment, John discovered a possessed woman writing something on a wall. After exorcising the woman, John saw that she wrote the words "Liv Die". Remembering his promise to his friend, Jasper Winters, John left the hospital and went to the United States. There he met Liv Aberdine, the daughter of his friend who (like her father) could see the true nature of supernatural things when in contact with an amulet. They soon learned that the demon Furcifer pursued and wanted to kill her. John decided to ask the help of Ritchie to defeat the demon. The three then prepared a trap for the demon. As John began to recite an incantation to send the demon back to hell, Astra appeared. Furcifer then made an offer to John: if John freed him then Furcifer would release Astra's soul from Hell. As John was about to free Furcifer, Liv discovered that it was not Astra, but a demon disguised as her. Learning this, John performed an exorcism and sent the demons back to Hell. Liv then left John to go and live with a friend, while John made the decision to help Manny to discover the nature of the thing that was causing the supernatural world to go wild.
The Rising Darkness
After defeating the demon, Lamashtu, John found out that the cause of all the supernatural events that have been happening were because of "The Rising Darkness". The rising darkness was a plan by the Brujería, to merge Hell and Earth together. The Brujería found a way to break the boundary that kept Earth and Hell separate. This would allow demonic creatures to roam freely across the land.
Restoring Sara Lance's soul
In late 2015, John received a call from his old friend Oliver Queen asking him to come to Star City to help him restore the soul of his friend Sara Lance which he accepted. He meets with Team Arrow and explains that Sara needed a restoration of her soul, not an exorcism as her sister Laurel mistakes it for. John also comments on the number of pretty girls Oliver has surrounded himself with which makes them feel somewhat uncomfortable. Much to Felicity's confusion he requests a peacock feather, which he only uses to scratch his back. John explains that the ritual will transport them to the "other side" where Sara's soul is trapped and transports himself, Oliver and Laurel. Upon entering the spirit world, in the shape of Nanda Parbat, John, Oliver and Laurel find the room where Sara is trapped in the Lazarus Pit. John holds off the spirit of a League of Assassins member, as Oliver and Laurel free Sara. All of them return safely with Sara restored to life. As John leaves, Oliver thanks him for his help but John also tells him he's had a dark feelings since being in town, Oliver confirms it as their newest problem Damien Darhk. John however warns Oliver to be careful around Darhk and advises he leave town, while he still can before departing.
In late February 2016, when Diggle suggested Constantine to help rescue William from Damien Darhk, Oliver mentioned that John wasn't an option because he was in Hell, presumably to fulfill his promise of saving Astra Logue.
By May, Oliver was finally able to get in contact again with Constantine, asking for a means to defeat Darhk's magic. In response, Constantine sent Oliver to meet with Esrin Fortuna, who had better understanding of Darkh's magic, and could possibly teach him to defend against it.
Having witnessed the horrors of the world John is very cynical and seemingly unconcerned with the value of human life, often deflecting his real feelings with sarcastic jokes even towards the possibility of his own death. Due to the number of people who have died around him John prefers to work alone and anyone he does work with, he avoids attachment so he won't be as hurt if they get killed. He is also known for having a very dry sense of humor.
John is also noted for being brutally honest with people, even angels, and never being shy of his opinion of the women he encounters. It's been shown a few times he openly expresses an attraction to Zed and after meeting Team Arrow he openly refers to all the women of the team, Laurel, Felicity and Thea, as "pretty girls". This also causes Thea to develop an attraction to him calling him a "specific kind of yummy", much to Oliver's discomfort. He also has a sarcastic personality, telling Oliver he would have dropped by sooner if he had known Oliver knew so many pretty girls.
Despite his apart bleak view of the world and cynicism John is a very honorable man as he is always there for the few friends he has, helping Chas to retrieve his daughter from Felix Faust and helping Oliver Queen to restore Sara Lance's soul. He also did a protection spell on Oliver despite his refusal to have attachments to people when he saw Oliver had a pure heart.
John tends to be a pessimistic realist as when Oliver thanked him for bringing Sara back he simply told Oliver he owed him only for Oliver to rebuttal that instead of that, they should always have each other's back when need be, not as business acquaintances, but friends.
Powers and Abilities
John is a master of the dark arts, and can utilize magic for various purposes.
John is an expert detective, as well as an expert in various magical and supernatural concepts. He also possesses some skill in swordsmanship.