Jason Blood was an immortal who lived thousands of centuries bound to the demon Etrigan. Making a deal not to kill himself to trap Etrigan in the lowest pit of Lucifer's Cricle, Jason got to know the Demon and after a while came to accept his situation, even though he knew the monster never could. His life constantly fluctuating, Jason always tried to do the best that he could in the situations that he was placed in.
After Jason of Norwich's father forced him to become a scribe of Camelot, Jason was put under the tutulege of Merlin. After this, Jason's father died leaving him orphaned. Jason of Norwich constantly chafed working under the grand mage, though he was the only one Merlin truly trusted. Jason also met the Fey, Nimue, and fell in love with her, though he deigned to confess it. Believing his potential to be greater than Merlin or even King Arthur let on, Jason tried his hand at magic but failed, sparking a rift between him and Merlin. Jason was betrayed by his mentor at the fall of Camelot and bound to the vile and vicious demon Etrigan, prematurely graying some of Jason's hair. Learning soon after that he could switch places with Etrigan at will, Jason also learned that doing so subjected him to torture in Hell.
His life now immortal, Jason and Etrigan travelled together for decades, getting to know each other well. The two roamed Britain, now full of civil wars, until Jason arrived to witness the witch burning of Madame Xanadu. Swapping places with Etrigan, Jason managed to save Xanadu, whom he discovered to be the new name taken on by Nimue. However, due to Etrigan also falling in love, surprisingly, with Xanadu. This prevented their marriage. Jason's vile acts across the nation with Etrigan had erased his past as a boy from Norwich, and resulted in him becoming Jason Blood. Journeying with Madame Xanadu for centuries, Blood, Xanadu and Etrigan and plenty of adventures, meeting Vandal Savage during the fall of Brigantia.
In the Dark Ages, Blood, Xanadu and Savage helped assemble a team to fight against the Horde. This team comprised of Sir Ystin, Al Jabr, Horsewoman and Exoristos and was soon coined the Demon Knights by Merlin himself. Blood's hatred for Merlin only increased during this time. Blood was not present for the defense of Little Spring against the Horde as for most of it, he had swapped places with Etrigan and endured tenure in Hell. For most of his stay in Alba Sarum, Jason stayed asleep before swapping again with Etrigan. When the two once again swapped, it was at sea, with Jason learning of the quest to go to Avalon. After once again swapping with Etrigan, the only times Jason swapped back was to use hellfire to break free of the holds of Morgaine le Fey. However, after Etrigan betrayed the Demon Knights, Jason confronted his worse half in the Battle of Avalon. After the battle, Merlin once again forcibly bound the two.
The time spent in Avalon shunted the group slightly forward in time and Jason and Xanadu were forced to separate. Savage, bitter at Etrigan's betrayal, captured Jason, had a wizard silence him, and tortured him at his fortress in Gibraltar. After thirty years Jason was rescued by most of his former companions and brought to Xanadu. Greatly angered at their request of Etrigan, Jason confronted Lucifer and began to torture the demon by refusing to return to Earth. Eventually, when the Vampiric Incursion of Themyscaria happened, Jason was forced to swap places. Nonetheless, when the Holy Grail was found, Jason had his revenge against both Etrigan and Savage, by ending his binding with the former and trapping the latter. Arriving at Al-Wadi, the city was attacked by the Giants, prompting the Battle of Al-Wadi. Knowing that he would be able to aid little in the battle, Jason was forced to call upon Etrigan. Although he stayed with Xanadu for a small while afterwards, eventually the love shared between the two would finally crack and the two separated.
Over a thousand years later, Jason, still bound to Etrigan, moved to Gotham City, a cesspool of violence practically tailor made for him. During his time in Gotham, Jason lost his memory, forgetting how to summon Etrigan and only remembering his name. Posing as a doctor and dyeing his white streak violet, in 2014, Jason made contact with a magical patient who made him remember his time with the Demon Knights. Returning home, Jason was confronted by Etrigan but due to his amnesia, was unable to make sense of the incident and was rescued by Batwoman. Shortly after another shift, Jason was again confronted by Etrigan, who was this time bound by Morgaine le Fey. This led to him joining another group, the Unknowns. After regaining his memories and being once again bound to Etrigan, Jason and the rest of the Unknowns parted ways.
Jason's most prominent power was his immortality. It granted him slightly expanded stamina, endurance and strength. He was also able to regenerate his wounds within an instant of getting them. Jason had one of the most powerful minds in the world, as he was fluent in nearly every language known to mankind and was able to devise devious plots. Although he had not sought Merlin's tutulege, his time spent around the great wizard's scrolls had enabled him access to certain spells. Nonetheless, he was not very experienced in the ways of magic. He was also able to access Hellfire due to his bond with Etrigan. Although it would have been easier to classify Jason as a Parahuman due to his longevity and association with magic, given that Jason was born centuries after the planting of the metagene, it was much more likely that he was a Metahuman. His strong resistance to pain and ability to bond with Etrigan were proof of this.