The myth of Niobe. From the Wikipedia: Niobe (Νιόβη) was a daughter of Tantalus and of either Dione, Eurythemista or Euryanassa, and the sister of Pelops and Broteas, all of whom figure in Greek mythology.
According to the accounts, Niobe boasted of her superiority to Leto because the nymph only had two children, the twins Apollo and Artemis, while Niobe had fourteen children (the Niobids), seven male and seven female. Using poisoned arrows, Artemis killed Niobe's daughters and Apollo killed Niobe's sons, while they practiced athletics, with the last to die begging for their lives. According to some versions, at least one Niobid was spared, (usually Meliboea). Their father, Amphion, at the sight of his dead sons, either killed himself or was killed by Apollo for having sworn revenge. Devastated, Niobe fled back to Mount Sipylus and was turned into stone, and, as she wept unceasingly, waters started to pour from her petrified complexion.