Genesis 4 opens the first family born outside Eden with two brothers already at odds. Cain, the elder, works the soil; Abel, the younger, keeps flocks. Each brings an offering, Cain from his harvest and Abel from the firstborn of his flock, and God regards Abel's offering but not Cain's. Genesis gives no stated reason for the difference, a silence that has driven interpretation ever since rather than settling it. Cain's face falls, and God warns him directly: sin is crouching at the door, eager for him, and he must master it.
He does not. Cain lures Abel into a field and kills him, the first death and the first murder in the narrative. God asks, "Where is your brother Abel?" and Cain answers with the question that has outlived every other line in the story: "Am I my brother's keeper?" God tells him Abel's blood cries out from the ground, curses the ground itself against him so that it will no longer yield its strength to his work, and sentences him to wander as a fugitive.
Cain protests that the punishment is more than he can bear, that he will be hidden from God's presence and killed by anyone who finds him. In response God places a mark on Cain, not as a further punishment but as a protection, and declares that whoever kills Cain will suffer sevenfold vengeance. Marked and spared rather than executed, Cain settles east of Eden in the land of Nod, builds the first city named for his son Enoch, and fathers a line that runs through Lamech, who boasts of his own violence multiplied seventy sevenfold. The text never says what the mark was, and no tradition has settled the question since.