According to the Vita Aeschyli (retrieved from link to the University of Bologna):
"τινὲς δέ φασιν ἐν τῇ ἐπιδείξει τῶν Εὐμενίδων σποράδην εἰσαγαγόντα τὸν χορὸν τοσοῦτον ἐκπλῆξαι τὸν δῆμον ὡς τὰ μὲν νήπια ἐκψῦξαι, τὰ δὲ ἔμβρυα ἐξαμβλωθῆναι. ἐλθὼν τοίνυν εἰς Σικελίαν Ἱέρωνος τότε τὴν Αἴτνην κτίζοντος ἐπεδείξατο τὰς Αἰτναίας οἰωνιζόμενος βίον ἀγαθὸν τοῖς συνοικίζουσι τὴν πόλιν."
I don't speak Ancient Greek, however Google Translate says it says:
"some of them were seen in the show of the Eumenides sporadically by introducing the dance, you astounded the demon as you chilled children, but embryos are aborted. when they came to Sicily they saw Hieron then the Aetna builder Aetnaia was shown, symbolizing good life to those who inhabit the city."
Professional translators your job is safe. Well we know they're talking about the playwright Aeschylus, and they're talking about the play The Eumenides
Queen Klytemnestra has murdered her husband King Agamemnon, in revenge for sacrificing their daughter. Under Greek law their son Orestes has to avenge his father or be cursed by the gods. However if he kills his mother he'll have killed his own kin and be destroyed by the furies. By the opening of the Eumenides, the final play in the trilogy, Orestes has killed his mother and her lover, avenging his father. However this means the furies are now hunting him to destroy him. Orestes calls on Apollo for help, Apollo sends him to the temple of Athena to face trial. Athena will eventually acquit Orestes and everyone goes home happy. Orphaned but happy.
Somewhere in there is the horror that, according to the Vita Aechyli, caused such terror in the audience that "embryos were aborted". It is reasonable to assume that the ancients considered The Eumenides a terrifying play.
The Vita Aschyli tells us that it was the introduction of the chorus that caused the terror. The play itself (retrieved from link to Project Gutenberg) tells us that the chorus played the furies. So it was the chorus, the singers and dancers, who scared the audience.)
I have to confess here that I'm not an expert on Greek theatre. I do however recall Medea murdering her own children, Oedipus gouging out his own eyes, Agamemnon (probably - Euripides' own ending to the play is pretty certainly lost) sacrificing his own daughter. In the Bacchae Pentheus is torn apart by his mother who takes his severed head home with her. Antigone is buried alive by her childhood friend, her brothers' corpses left to rot unburied (a true horror to the Ancient Greeks).
My point here is that Classical Greek theatre isn't nice. It's very gory and very scary. I would say it brags about it but the Vita Aschyli is the only written evidence I can find, however it's used enough times, by enough playwrights in what was after all a competition, to powerfully suggest that the audience didn't mind it at all. Even if it did cause spontaneous miscarriages. In fact I'd go as far as to say that they rather enjoyed it, enough for playwrights in a competitive environment to keep revisiting it.