Creon enters as a messenger from the prophet god (not sure), Phoebus.
He tells Oedipus that the plague is a result of the injustice done on
King Laius' murderer. Oedipus reacts by assuring the chorus (Theban
Elders) that he will help find the killer and serve justice as
necessary. This is ironic because Oedipus is, in fact, his father's
(King Laius) killer.
Actually, I find this quite Ironic. Because if you remove a period in
between a couple sentences on page 29 it reads, "Not merely from a
fellow-creature will I clear this taint, but from myself, the killer
of Laius..."
Without the period after 'myself', Oedipus would be clearly
identifying himself as the murderer of King Laius.