In Greek mythology, Xanthus or Xanthos (Ancient Greek Ξάνθος), son of Ptolemy, was the last sole king of Thebes in Pausanias' Boeotian roll. He dies in a treacherous duel — Pausanias names Andropompus; other traditions Melanthus — after which the Thebans abandon single kingship for shared magistracies.
Adapted from Wikipedia, "Xanthos (king of Thebes)" · CC BY-SA 4.0
I.Last sole king
Pausanias (9.5.16) closes the Theban man-king list with Damasichthon, Ptolemy, and finally Xanthus. When Xanthus fought Andropompus and was killed by craft rather than fair combat, the Thebans judged it better to entrust government to several people than to one man — the end of sole kingship at seven-gated Thebes in his reckoning.
II.Duel and Apaturia
A wider Attic tradition has Xanthus wage war on Athens and challenge Thymoetes; Thymoetes sends Melanthus (or Melanthus' father Andropompus), who tricks Xanthus by crying that a second fighter stands behind him in a black goatskin — identified with Dionysus — and kills him as he turns. Ionian cities later celebrated the victory in the Apaturia festival (Ephesus and Colophon excepted).
III.Homonyms
Not xanthus-phaenopid (Diomedes' Trojan victim), Scamander/Xanthus the river-god, Achilles' horse, or the other golden-named figures on Wikipedia's Xanthus (mythology) list.