Tereus was a Thracian monarch from Greek mythology.
Tereus is the son of Ares. At one moment, he was victorious on a great war. To thank him, Pandon gives him her daughter Procne as his spouse. However, he marriage is celebrated under bad auspices (an owl lands above the bridal chamber). They father a son, Itys. Procne asks her husband to see his sister, Philomela, and he goes to Athens for this purpose. As soon as he sees Philomela, Tereus ignites with desire for her and pays her an assiduous court. Pandion, oblivious to the situation, assignes Philomela with him, who wants to see Procne again; but as soon as they return, Tereus sequesters Philomela in a sheepfold, he rapes her then cut her tongue out and held her captive so she could never tell anyone. He told his wife that her sister had died.
Philomela wove letters in a tapestry depicting Tereus's crime and sent it secretly to Procne. During a bacchic party, Procne puts on a disguise and manages to find his sister. Realizing what Tereus has done, Procne killed her and Tereus' son Itys and served his flesh in a meal to his father Tereus who delights in it without knowing what it is. When he asks to call Itys, Procne replies: "You have in you the one you are asking for!". Philomela then appears and throws her son's severed head in his face. Enraged, Tereus tried to kill the sisters but all three were changed by the Olympian Gods into birds: Tereus became a hoopoe; Procne became the nightingale whose song is a song of mourning for the loss of her son; Philomela became the swallow. Because she has no tongue she can only twitter instead of singing.