Midrash for Genesis 4:24

The three remained where they were: Cain dead, the child slain, and Lamech blind. At nightfall his wives went out to search for them and found their grandfather dead, their son Tubal-cain slain, and Lamech. It was then that the earth swallowed up the four families: Enoch, Irad, Mehujael, and Methushael. Thereafter, Lamech served as the Angel of Death in fulfillment of the verse: Cain was avenged sevenfold, and Lamech seventy and seven (Gen. 4:24).