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 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.
Seems to be pretty strong.