One of the most wonderful and terrible things about reading Oedipus Rex by Sophocles is that we know what Oedipus does not. We know that he unknowingly killed his father; we know that he married his mother; we know that he is the brother-father to his own children; and we know that the prophet Teiresias is physically blind but speaks the truth to Oedipus. In this speech, we know that Oedipus means well but is, in fact, cursing himself without knowing it.
Things are bad in Thebes. Everything is dying and no one can figure out why. The people beg Oedipus to find out what has caused this curse and then do something about it. Oedipus sends Creon to the oracle to learn the cause of his city's troubles; Creon returns and says they are to get rid of the "polluting stain this land has harboured." Until then, Thebes will never be healed or relieved from its misery.
Creon goes on to explain that the "polluting stain" the city has been harboring is the unknown murderer of Lauis. Oedipus, not knowing that he is the one who committed the unholy act of killing the king, asks questions and tries to gather all the information he needs to find the murderer. Unfortunately, there is not much to be known. Creon says again:
Laius was killed. And now the god is clear:
those murderers, he tells us, must be punished,
whoever they may be.
Oedipus vows to uncover the mystery and find the murderer. He and Creon enter the palace, leaving the chorus to ponder what it has heard. Soon Creon emerges and delivers the speech found in lines 258-292. It is a proclamation, a threat, and a curse.
Oedipus says that anyone who knows who murdered Laius must reveal it to him immediately; if the murderer speaks up and confesses, he will be only exiled (sent away from the city) and not killed. If anyone knows who the murderer is, even if he is from another place, and tells Oedipus his name, he will receive the thanks of the king. If, however, anyone knows the truth and is afraid to speak it, Oedipus says he must treat the murderer as an anathema. Citizens are not to allow the murderer in their homes, they are not to talk with him, pray with him, or share religious rituals with him. Even more, Oedipus curses the murderer:
And I pray
whoever the man is who did this crime,
one unknown person acting on his own
or with companions, the worst of agonies
will wear out his wretched life. I pray, too,
that, if he should become a honoured guest
in my own home and with my knowledge,
I may suffer all those things I’ve just called down
upon the killers.
So, he warns his people not to harbor the murderer, shunning him in every way; then he heaps "the worst of agonies" on the murderer's head until the day he dies. In a final display of his seriousness about this matter, he ends his speech with the statement that if he has in any way been an unwitting accessory to harboring the murderer, he will suffer all the same curses he just called down on the murderer.
In three lines, then:
Anyone who knows anything about the murderer should speak up, and if the murderer confesses he will simply be sent away without punishment. Those who might know the murderer should ban him from their homes and shun him. If the murderer is discovered, he is cursed to a horrible life; if Oedipus has had any part in harboring the murderer, he will reap the same curses as the murderer.
Oedipus has just unknowingly cursed himself--twice.