|“||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.||”|
The shield was originally made by Hephaestus, using the head of Medusa as an imprint on the face after Perseus turned over Medusa's head to Athena.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 Castellan 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, which activates when you tap it and say "Aegis." Then, Thalia used the shield to scare a Leucrota so they could escape the house. Many demigods were lured there by the shield, but were trapped and killed.
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 Jackson, Annabeth Chase, 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.
During Reyna's fight with Orion, her bravery causes Athena to infuse Reyna's cloak with the power of aegis which Reyna remembers Athena has done to her favored heroes in the past. The power of aegis shields Reyna from an explosive arrow to the point that when the cloak covers the arrow, the explosion, which Orion believed would destroy the Athena Parthenos and a yacht, feels like a small pop like a helium balloon popping instead of a massive, deadly explosion. Athena sends Reyna a message about giving her aegis and Reyna uses it to great effect in battle: when Orion fires an arrow at her, she blocks it with her cloak and it shatters against it. With the strength of Bellona, Reyna is able to jump on Orion and strangle him with the cloak. As aegis it has Athena's power in it, allowing Reyna to kill the giant without a god physically being there to help her. Her cloak is described as having changed after having aegis added to it so now it has flecks of gold in it like it was laced with Imperial gold.
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. The part of aegis given to Reyna is able to absorb a deadly explosion so it feels like a helium balloon popping rather than a devastating explosion. An arrow from the giant Orion just shatters against it. As it is infused with the power of Athena, Reyna is able to kill the giant by strangling him with her aegis-infused cloak without Athena physically being there as before only a god and demigod working together could kill a giant.
- Only Thalia's replica shield has made an appearance in the books; the original Aegis, owned by Athena, is yet to be seen.
- When not active, Aegis becomes a silver charm bracelet.
- Aegis appears on the U.S. cover of The Demigod Diaries.
- Aegis is the name of the defense system used in modern US missile cruisers.