Cain was the brother of Abel and the first-born son of both Adam and Eve, and he had a rivalry with his sibling fuelled by jealousy that quickly turned to rage - his rage would then turn to sin as Cain killed his brother in a mix of anger and jealousy: according the Biblical teachings this was the first true murder mankind committed and as such Cain has become an immortal symbol of villainy in religious works and is referred to often as "the first murderer".
Cain's actions lead humanity further into darkness and in some texts Cain is seen as a son of the Serpent (also referred to as Satan) - even when seen as an ordinary man rather than a demonic figure, Cain is portrayed as corrupt and cruel - in some ways reflecting the darker aspects of humanity.
As punishment for his crime, Cain was cursed to wander the earth by God and bear a mark that forbade any man from killing him (upon Cain's protest to God that he would be killed in revenge by his fellow men) - the exact nature of this curse has been debated for centuries and varies considerably depending on the intereptation: although a popular legend that had its origins in medieval times was the idea of Cain's curse being an immortal one: much like the Flying Dutchman or the Wandering Jew.
Cain is a popular figure in fiction: often repeating the murder of his brother in many different settings - a famous example of Cain in fiction is the character in DC comics: who is portrayed as immortal and doomed to endlessly murder Abel over and over for all of time.
- Cain is sometimes considered the ancestor of evil and in some texts is seen as the actual son of the Serpent - though not all texts support such claims.