Aegis

"The shield has the head of Medusa molded into the bronze, and even though it won't turn you to stone, it's so horrible, most people will panic and run from the sight of it."

- Percy Jackson

Aegis is a shield that has the face of Medusa on it. A replica of the original shield is owned by Thalia Grace.

History
The shield was originally made by Hephaestus, using the head of Medusa as an imprint on the face after Perseus turned over Medusa's head to Athena. The shield itself was shared between Athena and Zeus after its' creation. Other gods have also been known to carry the shield, such as Apollo who Zeus sent to retrieve the injured Hector of Troy from the Achaeans.

The shield was used by the gods as a way to scare anyone that looked at it, as Medusa was able to turn anyone that looked at her to stone. Because of this, the

shield would radiate fear. Throughout history, other people (such as Alexander the Great) would imprint the face of Medusa onto their armor in the hopes of replicating the effects of Aegis.

The Diary of Luke Castellan
While Luke and Thalia were trapped in the house of Halcyon Green, he showed them a safe that was in his closet, which Luke opened, and found a bracelet. Thalia couldn't find out how to use it to start off but Luke figured out that it had a code word. Then, Thalia used the shield to scare a Leucrota so they could escape the house.

Percy Jackson and the Olympians
Thalia uses a copy of Aegis as her main shield in battle. When the shield is not active, it has the appearance of a charm bracelet which she wears all the time. When she taps the bracelet, Aegis comes out. Some demigods will not even look at the face of Medusa on the shield because it is so terrifying. It also sends monsters scurrying in fear as seen when Percy, Annabeth, and Thalia first encounter Dr. Thorn (a Manticore) in The Titan's Curse. In The Battle of Manhattan, the shield is seen to have an Aura that sends monsters and humans alike running away in fear.

Use
When activated, the shield gives off a terrible, fearful aura that forces adversaries away from the shield and the wearer. It provides extremely good safety for the wielder. It has shown to be extremely durable, more so than ordinary shields. This was shown when Thalia and Percy were battling the Manticore and defended against the monster's missile spikes. It held up without a single scratch while Percy's shield was heavily damaged and dented but then again, Percy's shield was made by Tyson when he was still a rookie blacksmith, and also a possible contributing factor is the fact that it was made to be collapsible, so it may not have been as durable as normal shields either.

Trivia

 * Only Thalia's replica shield has made an appearance in the books; the original Aegis, owned by Athena, has yet to be seen.
 * When not active Aegis becomes a silver charm bracelet.
 * Aegis appears on the cover of The Demigod Diaries.
 * Aegis is the name of the defense system used in modern US missile cruiser.