After Cain became the first murderer by killing his brother, he was not killed but cursed by God and driven into exile in the land of Nod. There he took a wife (a famously Unknown Character), and at least six generations were descended from him. Old Testament literalists point out that this means that there were humans not descended from Eden somewhere on Earth concurrently with the family of Adam.
Fictional depictions of Cain's descendants vary; they may inherit the wicked nature of their Biblical Bad Guy progenitor, perhaps to the point of having degenerated into hideous monsters. The mark specifically placed on Cain by God, which is associated with God's promise that sevenfold vengeance would be taken on anyone who would kill Cain, may or may not be an obvious physical feature inherited by his descendants.
Cain's descendants may have all been drowned in The Great Flood, whose survivors were all descended from the line that proceeds from Adam's other son Seth through Noah; this would necessarily limit depictions of Cain's descendants to antediluvian Bible Times. Some believe, however, that some of Cain's descendants survived the Flood, and still bear the identifiable mark of their ancestor.
Cain as character has his own trope here. See Nephilim for another group of Biblical descent which may have some crossover. Compare Lilith. A subtrope of Sins of Our Fathers, in which entire Always Chaotic Evil species are the result of Cain's infamy.
This trope does not apply to those descended from anyone else who has the Name of Cain.
- In the DC Universe, Vandal Savage was at one point revealed to be the Biblical Cain, leading to stories in which the monster Grendel, his daughter Scandal Savage, and his indirect descendant Batwoman had to deal with the familial and mystical consequences of being, well, descendants of Cain. (This was later retconned by making the Mark of Cain transferable, with Savage stated to not be its first bearer.)
- The Hellblazer "Third Worlds" story arc has John meet with a tribe consisting of Cain's descendants, living near the entrance of the Garden of Eden in the Middle East in a state of perpetual penance. Their religious zeal leads his current girlfriend to mistake them for Islamists.
- Werewolves in Crimson were descended from Cain, following his murder of Abel whose circumstances are so much different in the Bible: he used an angelic sword to kill his brother under its influence, not because he was envious of God's favor over his brother.
- Vampirella is revealed in the 2014 Dynamite comics to be the daughter of Cain. In addition to being cursed with immortality, he also would always have twin children (one dark-haired and the other blonde) and they would be driven to kill each other, with the blondes being the ones to go evil and murder their siblings. Throughout the ages, Cain saw his family being destroyed so many times as part of his curse and true to that, Vampirella's twin sister Draculina also wants to kill her.
- Noah has the antagonists explicitly be the descendants of Cain. Not all of them are evil, they "merely" suffer from a near total cultural lack of the wisdom and compassion needed to use their knowledge and technology to live in harmony with nature— and each other. This has gone on since the beginning of their history, leading to ravaging the planet to the point of apocalyptic resource depletion and descent into cannibalism. Of the Cainites, only Ila and the girl Ham finds, Na'el, were shown to be innocent, and the former was basically raised by descendants of Seth.
- In Beowulf, it is said that the monster Grendel and his mother are descended from Cain, alongside "ogres and elves and evil phantoms" and "giants".
For the killing of Abel
the Eternal Lord had exacted a price:
Cain got no good from committing that murder
because the Almighty made him anathema
and out of the curse of his exile there sprang
ogres and elves and evil phantoms
and the giants too who strove with God
time and again until He gave them their reward.
- A positive example in Hermann Hesse's Demian, where the title character speculates that Abel's descendants made the story up out of resentment, because some people are more intelligent, insightful, and ambitious than others. Demian, incidentally, has a certain 'brightness' on his forehead which the protagonist mentions several times.
- In the Sabina Kane series, the vampires believe that they are descended from a love affair between Cain and Lilith. All vampires are redheads, which is a relic of the Mark of Cain.
- In The Well of Loneliness, the "inverts" are often ironically referred to as those bearing the Mark of Cain, in keeping with the religious themes of the novel.
Dean: A bloodline?
- In "The Song Remains the Same", Michael says the Winchester bloodline is descended from Cain and Abel. Their descendants are the only ones who can be used as vessels by the Archangels Michael and Lucifer to manifest on Earth.
Michael: Stretching back to Cain and Abel. It's in your blood, your father's blood, your family's blood.
Dean: Awesome. Six degrees of Heaven Bacon.
Cain: At most, I'm culling...one in ten.
- However, Cain's entire backstory was changed in "First Born", where they made him a reformed demon after he slaughtered Abel for being tempted by Lucifer. "The Executioner's Song" builds on this retcon by having Cain actively hunt down and kill his descendants in order to remove his evil from the Earth, and as a Continuity Nod to the above, he tells Castiel that he'll get to Dean (and presumably Sam, though it's not stated explicitly) in due time.
Castiel: Of everyone?
Cain: I've got time.
- The Bible names six generations of Cain's descendants. Cain begat Enoch, who begat Irad, who begat Mehujael, who begat Methusael, who begat Lamech. Lamech's wife Ada bore him Jabal the shepherd and Jubal the musician; their half-brother Tubalcain and half-sister were Lamech's sons by his other wife, Zillah. Lamech explains to his two wives how, where Cain would be avenged sevenfold, Lamech's vengeance is seventy-sevenfold. According to Jewish tradition, that daughter of Lamech, who was named Naamah, was Noah's wife—meaning that you (and everyone else) are a descendant of Cain.
- The idea that Cain or his descendants were cursed with black skin dates back at least to the Middle Ages. Many Christians in the eighteenth and nineteenth centuries also believed that dark-skinned people were the descendants of Ham and a wife who was descended from Cain. Thus, they attributed to black people not only the mark of Cain but the curse God placed on Ham's son Canaan that declared him "servant of servants." As one can probably imagine, this was used by many racists in the Americas to justify the enslavement of these people. This is usually called the "Curse of Ham".
- The Mormon scripture The Pearl of Great Price describes a vision in which "the seed of Cain were black" and thus set apart from all of Adam's other descendants (Moses 7:22). The scripture also states that Canaan's children were all cursed with a despicable "blackness" (Moses 7:8), and explains that the Egyptians were forbidden the priesthood because their Canaanite descent through Ham's daughter Egyptus "preserved the curse in the land," apparently referring to the curse of Ham (Abraham 1:23—27). Though the scripture neither claims that Cain was punished with blackness nor identifies him as an ancestor of the Canaanites or the Egyptians, Brigham Young's non-canonical Journal of Discourses explicitly associated black skin and the curse of Ham with the descendants of Cain: the postdiluvian "servant of servants" curse was "pronounced upon the same race" as Cain, whose mark "is the flat nose and black skin," and whose curse will not be lifted "until all the other descendants of Adam have received the promises and enjoyed the blessings of the Priesthood and the keys thereof." In part because of teachings like these, the Church of Jesus Christ of Latter-Day Saints barred people of African descent from the priesthood (though at least one had been ordained under Joseph Smith) until 1978, when a new revelation received by the Church leadership allowed "all worthy males" to be ordained to the priesthood from that time forward.
- Another sectarian belief is the "serpent seed" doctrine, in which Cain, the first child of Eve, was actually not the first child of Adam, but rather of Satan, with whom Eve had intercourse with in the Garden of Eden, and thus from this sex was considered the Forbidden Fruit and also that Sex Is Evil. Somehow in this doctrine Cain's descendants (referred to as the "serpent seed") survived the Flood and thus became the Jews, which explains Jesus saying to them in the gospel of John that "your father is the devil" and "your father was a murderer from the beginning." As you'd imagine, this is mostly used by highly antisemitic groups, and most Christians reject it. Even some Jews held it, but didn't claim descent from Cain (as you'd expect).
- Assassin's Creed II has one glyph puzzle reveal that The Knights Templar descended from Cain, and that their symbol (the Templar Cross) was Cain's mark. However, this is also not depicted as a biological descent, since the human race intermingled freely in the aftermath of the global firestorm. It is more of a philosophical descent, as evidenced by the Templar codes of conduct and willingness to kill the descendants of Assassins that have nothing to do with their conflict. Since Adam and Eve were real people in this continuity, Cain may have been a real person, too, but he couldn't have been the progenitor of the entire Templar Order, since several Assassins defected to join the Templars over history.