Aegis

"Her shield is modeled after one her dad uses-also called Aegis--a gift from Athena. 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 Thalia's shield, modeled after the actual Aegis, the shield of her father Zeus, which had Medusa's head imprinted on it after Perseus turned over Medusa's head to Athena, who placed it into the Aegis. When Thalia's 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 won't 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 encountered Dr. Thorn (a Manticore) and the Battle of Manhattan it had an Aura that sent the 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 very 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.