Which Greek seer interpreted the plague?

The Greek seer who interpreted the plague in the Iliad was Calchas.

Calchas was an Argive seer serving the Achaean army. Homer describes him as having knowledge of the past, present, and future through Apollo, making him the authority the Greeks consult when the army suffers a devastating plague.

At the opening of the Iliad, Calchas explains that Apollo sent the plague because Agamemnon had dishonored Chryses, the god’s priest, by refusing to return his captive daughter Chryseis. Calchas says the only remedy is to give Chryseis back without ransom and perform the proper rites. Agamemnon reluctantly agrees, but demands Achilles’ captive Briseis as compensation, triggering the quarrel that drives the epic’s central plot.

Calchas is sometimes confused with Tiresias, another famous Greek prophet, but Tiresias belongs chiefly to Theban legends and the Odyssey’s underworld episode. Calchas also appears in traditions surrounding the expedition to Troy, including the prophecy that Agamemnon must sacrifice Iphigenia to secure favorable winds.