Thalia's Shield is a replica of Aegis owned and used by Thalia Grace, the daughter of Zeus and the lieutenant of the Hunters of Artemis. It was found by Luke Castellan in the house of Halcyon Green. Thalia typically pairs it up with her spear.
It is often simply referred to as Aegis despite only being a replica. When not in active use, the shield takes on the form of a bracelet that is activated by the user tapping it and saying "Aegis." Like the real Aegis, it has the face of Medusa on the front, but as it is not the real Medusa's face, it does not have the power to turn others to stone. However, as a result, it tends to be frightening to enemies and has an aura that sends monsters running away in fear. 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 Wristwatch Shield was heavily damaged and dented.
At some point before Halcyon Green was born, the shield was locked in a cursed safe in his childhood home. After Halcyon was cursed by Apollo for using his prophetic abilities to tell a young girl the future, many demigods would visit his house in search of the treasure within, but would be killed by the safe's defenses or the leucrotae.
The Diary of Luke Castellan
Luke Castellan and Thalia Grace are led to Halcyon Green's house by Amaltheia where they become trapped. Hal leads the two to the safe, having believed that they were another treasure seeker after the contents. Using his powers as the son of Hermes, Luke is able to safely open the safe and retrieve the bracelet which Thalia believes Zeus had purposefully led her to. After remembering the story of Aegis, Luke suspects that Amaltheia, bitter after being skinned by Zeus and then reforming from Tartarus, had led them to the house to kill them in revenge for Zeus' actions. However, Thalia can't figure out how to activate the shield.
Later, as Hal destroys the house with Greek Fire, a surviving leucrotae attacks Luke and Thalia. Remembering the story surrounding Amaltheia and Aegis, Luke has her tap the bracelet and call out Aegis. The shield activates and when Thalia turns the representation of Medusa on it towards the monster, the leucroate is so frightened that it backs into the house's deadly drapes which consume and destroy it. After escaping from the house, Thalia returns her shield and spear to their deactivated forms.
Thalia wields the shield during the Battle of Manhattan.
Thalia uses her shield in battle against the forces of Commodus. After seeing the face of Medusa on the shield, Apollo realizes that it's not the original Medusa as that would turn him to stone, but the replica is so good that it scares Commodus' Germani into cowering away.