Tithonus is the Trojan prince beloved of Eos, goddess of the dawn — son of Laomedon (usually by the naiad Strymo) and brother of Priam. When Eos asked Zeus to make him immortal she forgot eternal youth; he withered into endless old age and, in later tellings, became a cicada. By Eos he fathered Memnon and Emathion, kings associated with the Ethiopian line at Troy.
Adapted from Wikipedia, "Tithonus" · CC BY-SA 4.0
I.Family and the Dawn
Tithonus was a prince of Troy, son of King Laomedon by the naiad Strymo in the usual stemma, and sibling of Priam and Hesione among others. Eos took him from the Trojan house as her lover; Homer already has Dawn rise from Tithonus's bed to bring light to mortals and immortals. By her he fathered Memnon and Emathion. One Apollodoran fork instead makes Tithonus son of Cephalus (another lover of Eos) and father of Phaethon — a strand the atlas does not merge with the Trojan prince.
Attic vase-painters of the fifth century sometimes show him as a rhapsode with a lyre in Eos's pursuit.
II.Immortality without youth
In the Homeric Hymn to Aphrodite, Eos asked Zeus to make Tithonus immortal but forgot to ask for eternal youth. He lived forever, yet when loathsome old age pressed upon him and he could no longer move his limbs, she shut him in a chamber behind shining doors, where he babbles endlessly without strength. Later tellings metamorphose him into a cicada (tettix), still living and begging for death — an aetiology for the dawn chorus of cicadas and for old men's high-pitched talk.
Hieronymus of Rhodes shifts the blame to Tithonus himself for requesting immortality but not agelessness. Propertius has Eos still embrace the aged lover rather than abandon him. Diodorus adds that Tithonus founded Susa in the east and was bribed with a golden vine to send Memnon to Troy.