Mist

The Mist is a type of magic or force that stops mortals from seeing monsters, gods, titans, and various other mythical creatures and supernatural occurrences. This includes seeing weapons, as in The Lightning Thief, when Percy had Riptide, and the mortals described it as a short gun.

The Mist replaces these things with visions of (relatively) ordinary happenings. Under the influence of Mist a mortal would see gods or monsters as ordinary humans or animals, and supernatural objects also become commonplace items. Some animals, such as rats, and clear-sighted mortals such as Rachel Elizabeth Dare, Sally Jackson, Ariadne and small children (but their parents never listen to what they say) can see through the Mist. Half-Bloods and mythical creatures such as satyrs can all see through the Mist at most times (though sometimes the mist is strong enough to fool demigods) and manipulate it to a certain degree, with Thalia and Chiron as excellent examples. although we do not know how powerfull the mist is we see in the demigod files that it can make Mrs. O'leary look like a poodle yet satyrs and centaurs must disguise themselves as opposed to go around as is and have the mist alter mortals perception. Gods are completely immune to the effects of the Mist. According to Chiron, the Mist is mentioned in the Iliad. It is not specified which part he is referring to, but Book XX of the Illiad mentions Poseidon pouring mist over the eyes of Achilles to protect Aeneas. Book V also specifically describes Athena giving Diomedes the ability to see gods:

"And I've removed the filter from your eyes, which covered them before, so now you'll easily distinguish between gods and men."