In Greek mythology, Nycteis (Ancient Greek: Νυκτηΐς) was a Theban princess, the daughter of Nycteus. She married Polydorus, the son of Cadmus and Harmonia and a king of Thebes, and by him became the mother of Labdacus, who in turn became king of Thebes. Through her, the line of Cadmus passed into the Labdacid kings of the city.
Adapted from Wikipedia, "Nycteïs" · CC BY-SA 4.0
I.Family
Nycteis was the daughter of Nycteus, a figure of the Theban royal house. Wikipedia, following later genealogical tradition, also names her mother as Polyxo.
She married Polydorus, a king of Thebes and the son of Cadmus and Harmonia, the founders of the Theban dynasty. By Polydorus she became the mother of Labdacus, who succeeded to the throne of Thebes and gave his name to the Labdacid line of Theban kings.
II.Mythology
Nycteis appears in the genealogies of the Theban royal house as the link between the house of Cadmus and the later Labdacid kings. Her marriage to Polydorus, son of Cadmus, joined her father's family to the founding dynasty of Thebes, and her son Labdacus carried that combined lineage forward.
Through Labdacus the line descended to Laius and ultimately to Oedipus, so Nycteis stands at an early point in the succession that produced the most famous of the Theban kings.
III.The Theban succession
After the death of Polydorus, his and Nycteis's son Labdacus was still a child, and the affairs of Thebes fell to others to manage on his behalf. The guardianship and regency of the kingdom passed within Nycteis's own family, to her father Nycteus and afterward to Nycteus's brother Lycus.
Labdacus eventually came of age and ruled Thebes in his own right before he died, leaving his own infant son Laius as heir. In this way the kingship traced through Nycteis remained at the center of the Theban dynastic story across several generations.