Introduction
Lamech, a descendant of Cain through Methushael, is remembered in Genesis for two notable details: he is the first person the text describes as marrying two wives, Adah and Zillah, and he speaks a short, boastful poem to them about violent vengeance, one of the oldest pieces of poetry preserved in the Bible.
Note: this Lamech, of Cain's line, is a different figure from the Lamech who is Noah's father in the Sethite line; the two share a name but are distinct individuals in the text.
Family
Two wives
Genesis 4:19 states that Lamech took two wives, Adah and Zillah, the first instance of polygamy recorded in the biblical text, presented without explicit comment or judgment in the narrative itself.
The Song of the Sword
Lamech speaks a short poem to his wives (Genesis 4:23-24), boasting that he has killed a man for wounding him and a young man for striking him, and declaring that if Cain is avenged sevenfold, he, Lamech, will be avenged seventy-sevenfold. This "Song of the Sword" is regarded by scholars as one of the oldest surviving fragments of Hebrew poetry, and its boastful tone of escalating violence is often read as illustrating the spread of corruption in Cain's line, in contrast to Seth's line.
Children
Lamech's children by Adah and Zillah, Jabal, Jubal, Tubal-cain and Naamah, are credited in the text with founding, respectively, the pastoral life of tent-dwelling herders, the playing of the lyre and pipe, the forging of bronze and iron tools, with Naamah named without a specific credited innovation.