Cain (カイン Kain) is the son of Adam and Eve, the first murderer in mankind's history. After he killed his brother, god cursed him with immortality. The earth kept rejecting his return (to die) and the blood and tears he shed gave birth to numerous demons. Detesting the earth, he also gave birth to conflict and culture. From there, human began to learn magic and education. As time passes, people started defying the logic of earth and created a man-made city. Up until now he's still dreaming for revenge against the world.
Cain was supposedly the son of Adam and Eve, the first human fashioned by human hands and for the crime of his existence he was banished outside of the world, branded a murder and cursed with immortality.
As the world rejected him he began to reject the world, alone in the darkness his power seeped through to the world and gave birth to all Demonkind. His influence brought civilisation and war to the world. Through him they learnt sorcery and and fashioned weapons.
He was banished to the world of Nod, a place in which no magic exists. In fact it is better to say that Nod cancels out any magic that encroaches on it.
Cain is only rumored to exist in legends that is until the fourth primogenitor's existence came to light as the fourth primogenitor is a weapon that is created to fight him.