Who is the one character in the play Oedipus the King, who saw, right from the start, the cause of the plague? Why is his identity ironic?

The one
character in the play who sees, right from the start, that the cause of the plague is 's
transgression in killing his father and marrying his mother, is Teresias.  Theof this is that
Teresias is blind, yet he alone can see the truth.

Teresias appears in the
first act of the play, having been summoned by the King to tell what he knows about the
calamities that have beset the land.