Mist

The Mist is a supernatural force controlled by Hecate that twists a mortal's sight from seeing monsters, gods, Titans, and various other mythical creatures and supernatural occurrences by replacing them with things the mortal mind can comprehend. This includes seeing magical weapons, as in The Lightning Thief, when Percy had Riptide, and the mortals described it as a rifle.

Description
It is Hecate's job to make sure the mythological events are replaced with visions of normality. Under the influence of Mist, a mortal would see gods or monsters as ordinary humans or animals, and supernatural objects also become commonplace items. Demigods 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 examples. Although the true extent of the Mist's power is unknown, 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 going around as is. Mist is strong enough to wipe from the memories of both the Greek and Roman demigods about the existence of each other. Thalia is able to manipulate the Mist, along with Chiron and probably many others.

In the short story Son of Magic in The Demigod Diaries, the Mist is shown to be able to form into physical objects by Hecate and her children. Alabaster Torrington created an Imperial Gold weapon, and a guardian from Mist as Mistforms, and Hecate created a Mistform body for the soul of Howard Claymore, who died to protect Alabaster. The Mist is also strong enough to distort mortals enough that they don't even realize that hazardous and deadly things are happening to them. Dr. Claymore's friend, Burly Black, burned to death without realizing a thing.

According to Chiron in The Lightning Thief, 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."

After that, Diomedes injured Aphrodite in the hand when she was trying to rescue Aeneas.

Another story about Jason and the Argonauts on their quest to recover the Golden Fleece has Hera cover the group in a layer of Mist that makes it so no one notices them until they reach the palace of King Aeëtes of Colchis, Medea's father.

Mortals With Clear Sight

 * Rachel Elizabeth Dare
 * Sally Jackson
 * May Castellan
 * Ariadne
 * Maria di Angelo
 * Esperanza Valdez
 * Younger children are able to see through the Mist
 * Some parents of demigods are able to see through the Mist, while others (like Tristan McLean) are unable to.

The Lightning Thief
The Mist is mentioned several times, normally hiding Percy's sword from mortal eyes. It also hides monsters, such as when the Furies attacked Percy and his friends on a bus, but the people only saw Percy attacking three old ladies. The Mist also hides most of Percy's fight with the Ares, making their swords appear as guns to the cops and also hiding that Ares was a god. The Mist also hides Camp Half-Blood from the eyes of mortals, as anything inside the Camp Borders appear as nothing more than a strawberry field.

The Sea of Monsters
The Mist made a group of Laistrygonian Giant appear as normal students until they showed their true form when they attacked Percy. The Mist also hid the fact that Tyson was a cyclops until Percy looked directly at him. Later when given a quest into the Sea of Monsters, it is explained that there is a strong layer of Mist in the Bermuda Triangle, resulting in ships and plains getting lost inside it. Chiron later uses the Mist to make up a fake cover story to shift the blame away from Percy for the attack on the gym.

The Titan's Curse
Thalia Grace showed that she is able to manipulate the Mist, as shown when she snapped her fingers and a teacher thought they went to the school. Later, as the group climbed up Mount Othrys, Thalia explained that the top of the mountain had a very strong layer of mist. Percy then asked if she meant the magical kind or the natural kind, she explained that both forms were strong. This may also indicate the strength of the Mist in California due to the existence of Camp Jupiter in the vicinity (particularly San Francisco).

The Battle of the Labyrinth
The Mist is very strong inside the Labyrinth. It is strong enough to fool demigods into traps or to get them lost. Percy eventually had to ask Rachel Elizabeth Dare for help, as she was a clear sighted mortal and could see through the Mist in the maze.

The Last Olympian
The Mist hid Typhon's true form from the eyes of demigods and humans. Regular humans saw it as a freak storm system that was moving across the country toward New York City, although Demigods such as Percy Jackson could see part of his form.

The Lost Hero
The Mist causes many false memories of Jason Grace with Piper McLean and Leo Valdez. However, their false friendship becomes a reality, with Piper's love interest with Jason eventually becoming true. It is also revealed how the gods used the Mist to divide the two camps.

The Son of Neptune
Even in Alaska, the land beyond the gods, the Mist is very strong. Hyperborean Giant live freely there, but most mortals simply ignore them and act like they aren't there.

Trivia

 * The Mist divided the Roman and Greek demigods for years - implicated by the strong presence of the Mist near Camp Jupiter.
 * Mortals who are a parent of a demigod usually have the ability to see through the Mist (like Sally Jackson), but not always.
 * The Mist cannot only distort vision, but also creates false memories and suppresses real ones.
 * The Mist does not completely hide some of the physical features of Satyrs/Fauns, leading them to find numerous ways to disguise themselves.
 * The Mist distorts weapons, monsters, battles, and places that are incomprehensible to mortal minds.
 * It is noted that another character saw flying horses near the Empire State Building or Olympus. It is still unknown if the Mist does not work for him because he has magician's blood or it is just his imagination.
 * In The Stolen Chariot, a mortal can see the horses that pull Ares' War Chariot. It is possible that she can see though the Mist.
 * So far, only females humans have been shown to be able to see through the Mist. It is unknown if male humans are unable to, or if none have been introduced in the series.