[From the Introduction to the Scholia to Lykophron’s Alexandra]
“A summary is as follows. Priam, the son of Leukippê and Laomedon, fathered twin children with Hekabê, the daughter of Dumas or Kisseus, Kasandra [and Alexandra] and Helenos, whom they took to the shrine of Helian Apollo in Thumbraion where they made the sacrifices for the occasion of their birth. After they drank together and celebrated all day in the temple, by nightfall they returned to the city and the palace, secretly leaving their children behind them in the temple, something they did (as far as I can see) according to custom to discover this: so they might know from the events what kind of people their children would be. [In the same way, at any rate, had those people around Priam done this concerning what was fated]. When they approached the temple on the next day, they discovered two snakes watching over their children and [purifying their senses?]. but they were not harming them at all.”
ἡ δὲ ὑπόθεσίς ἐστι τοιαύτη· Πρίαμος ὁ Λευκίππης καὶ Λαομέδοντος ἐξ ῾Εκάβης τῆς Δύμαντος ἢ τῆς Κισσέως θυγατρὸς διδύμους παῖδας γεννᾷ, Κασάνδραν τὴν καὶ ᾿Αλεξάνδραν T καὶ ῞Ελενον, οὓς ἐν τῷ τοῦ Θυμβραίου ναῷ ῾Ηλίου ᾿Απόλλωνος ἐκόμισαν, καὶ τὰ τούτων ἐτέλει γενέθλια. συμποσιάσαντες δὲ καὶ κωμάσαντες πανημερίως ἐν τῷ ναῷ περὶ ἑσπέραν πρὸς τὴν πόλιν καὶ τὰ ἀνάκτορα ὑπεχώρησαν λήθῃ κατά τινας ἐάσαντες τοὺς παῖδας ἐν τῷ ναῷ κατ’ ἐμὲ δὲ οἰκείᾳ γνώμῃ τοῦτο σκοποῦντες ἐξ ἔθους *κρατοῦντος* ἐποίησαν· ἔκ τινων γὰρ συμβάντων ἐπεγίνωσκον ὁποῖοί τινες ἀπο-βαῖεν οἱ παῖδες. οὕτω γοῦν καὶ οἱ περὶ Πρίαμον περὶ τοὺς εἰρημένους ἐποίησαν. T τῇ ἐπαύριον δὲ τῷ ναῷ προσελθόντες β′ ὄφεις ἐπῃωρημένους τοῖς παισὶν εὗρον καὶ τὰ αἰσθητήρια τούτων καθαίροντας, μηδὲν δὲ λυμαινομένους αὐτούς.
The end of the introduction (likely another author) provides a summary of the more common account:
“The myth is this: Apollo lusted after Kasandra and pursued her. She was caught by him and asked if he would give her the power of prophecy if she promised to marry him. The god made his promise, but she had deceived him about the marriage. This is why the god got angry and when he could not withdraw the prophetic power because it was a divine gift, he made her disbelieved when she prophesied true things—she seemed like she was mad!”
ἡ δὲ ἱστορία τοιαύτη· ᾿Απόλλων ἐρασθεὶς Κασάνδρας ἐδίωκεν αὐτήν. αὕτη δὲ ὑπ’ αὐτοῦ καταλαμβανομένη †T ᾐτήσατο, ἵνα δῷ αὐτῇ τὴν μαντικήν, ὑποσχομένη τὸν γάμον· τοῦ δὲ θεοῦ παρασχόντος τοὺς γάμους αὐτῷ διεψεύσατο, ὅθεν ὀργισθεὶς ὁ θεὸς τὴν μαντικὴν *ὡς* θεῖον οὖσαν δῶρον ἀπ’ αὐτῆς οὐκ ἀφείλατο, ἀπιστεῖσθαι δὲ αὐτὴν ἐποίησε μαντευομένην τὰ ἀληθῆ καὶ ὡς μαινομένην ἡγεῖσθαι.