Zeus sends a deceptive dream

I was just asked the following question,  "Can a dream ever be misleading or false? Or are dreams always based in truth, vision and foreshadowing?"

    My immediate response: With dreams, as with anything else, we want to check on the reliability of our sources, trust our feelings, test and verify and apply that rare commodity, common sense!
    The dream archaeologist in me then recalled a most instructive story about deceptive dreams from the Iliad..

     For the early Greeks, Homer was the closest thing to the Bible. One of the things they learned from him was that the powers beyond ordinary humans speak through dreams, but can also use dreams to transmit deceptive messages. And that we want to check what is behind the mask of a dream messenger. A familiar face may be a disguise, and we want to grasp the motives and agenda of the guiser.

    In a scene in Book II of the Iliad, Zeus decides to avenge the honor of his protege Achilles, who is sulking in his tent, by making it clear to the Greeks that he is the indispensable hero. Zeus lays a trap for Agamemnon, the leader of the Greek host, who has dishonored Achilles. 

    Zeus summons Oneiros and orders him to deliver a misleading message to Agamemnon. The name Oneiros means "Dream". Here a dream is actually a dream messenger, an independent entity. 

   To carry out Zeus' command, Oneiros puts on the semblance of Nestor, a trusted comrade of Agamemnon, and visits the sleeping king in this form. Standing over Agamemnon's head, the dream visitor tells him - quoting Zeus himself -that the gods are no longer taking sides in the war. Therefore the  Greeks should make haste to attack Troy, which will fall easily.

    Trusting the dream, Agamemnon recounts it to his battle captains, and they launch their attack - only to find that the walls of Troy are not easily breached, and they cannot succeed without making amends to Achilles and bringing him back into the fray.

    We understand from this tale the sources of the Greek suspicion of oneiropompoi, or "dream senders". These might be sorcerers who were adept at projecting thoughts at a distance. There are many stories in later Greek literature about human "dream senders" who abused their psychic skills in this way.
    We see that in dreams, as in other situation, we want to check the reliability of our sources.

    In his treatise on divination, Cicero observed of Homer's depiction of dreams, , “although these stories were made up by a poet, they are not far from the usual matter of dreams.” 

Zeus sends a deceptive dream, by John Flaxman (1793)