Origin
Jason o’ the Blood was a nobleman of the 6th century Camelot during the time of King Arthur. When Camelot was under siege by Morgaine Le Fey, the wizard Merlin summoned his half-brother demon Etrigan to help fight in the battle. Merlin eventually sent Etrigan back to hell, but in order for him to be summoned again, bonded him with the human Jason Blood which lasts to the present day.
Jason discovered a poem in crypt of Merlin which allowed him to summon Etrigan, which ended in the lines “Gone, gone the form of man, Rise the demon Etrigan!”
Creation
Jason Blood first appeared in The Demon #1 written and drawn by Jack Kirby.