Calchas rose among them,
Thestor’s son, the clearest by far of all the seers
who scan the flight of birds. He knew all things that are,
all things that are past and all that are to come,
the seer who had led the Argive ships to Troy
with the second sight that god Apollo gave him.

– Homer

The Iliad, Book 1, lines 79-84. Calchas is a very valued seer in the Greek army who watches birds and other signs of the will of the gods that controls the fate of humans. He guides the Argive ships to Troy and makes a major contribution to the war effort. The god Apollo has blessed him with second sight.