Women of Trachis by Sophocles (circa 430?) as part of my journey through Harold Bloom's western canon list, and I think it might be one of my favorite very old plays ever.
Here Sophocles tells the story of Deianeira, the long-suffering wife of Heracles (yes, that guy, the one the Romans called Hercules). She's been basically abandoned and raising their children while Heracles goes off to war and has adventures. She hears that her husband is finally coming home, but in the group of women prisoners from the city he sacked is one beautiful well-born lady, Ione, and Deianeira finds out that Heracles brought her back to take as a mistress. Deianeira tries to play it cool, but is feeling insecure and old and lonely and ends up using a love potion she got from a centaur to try and keep Heracles eye from straying. The potion plan goes horribly awry and after a bunch of death and some exceedingly amazing tragedy, things end and no one is happy.
The translation I read is extremely crisp and modern-feeling, but other translations I looked at online have much of the same feel to them, so some of that has to come from the original. I love that this play is almost entirely told by a woman, Deianeira, and a chorus (the titular Women of Trachis), and the usual hero, Heracles, doesn't come in until the final section of the play (and isn't all that heroic when he does arrive). And the tragedy, my God, the tragedy! This is one that everyone should read, and one that I'm definitely going to read again.
[Interested? Read the whole damn thing for free here, because very old plays are totally in the public domain. Just try reading the first monologue and see if you can stop yourself from reading the whole thing. Go on, I dare you.]