Original: The story of Niobe, and especially her sorrows, is an ancient one. The context in which she is mentioned by Achilles to Priam in Homer's Iliad is as a stock type for mourning.
Simplified: The story of Niobe is an ancient one among Greeks: Niobe is mentioned by Achilles to Priam in Homer's Iliad book XXIV, as a stock type for mourning.