Caligula is an Roman Emperor and one of the three main antagonists of Rick Riordan's series The Trials of Apollo, being one of the three members of Triumvirate Holdings who controls the western part of the United States.
As it was stated that the Triumvirate helped Kronos in the Percy Jackson series, and Octavan in the Heroes of Olympus series, Caligula is an overarching antagonist of the Rick Riordan universe. He is also the only villain to successfully and permanently kill a main character, Jason Grace.
Caligula is extremely arrogant and considers himself to be a god. He models himself after Apollo and tries to be like him. He is sadistic and enjoys inflicting pain. He believes that people should be taken for their word. For example, when an aristocrat declared himself Caligula's "servant", Caligula had him turned into a slave.
Caligula received his nickname as a child, as it meant "little boot", though he hated that nickname. He became emperor after he had his uncle Tiberius Caesar assassinated. As Emperor, he was power-hungry and sadistic, and enjoyed watching torture and executions. He tried to make himself into a sun god like Helios and Apollo. Eventually, he was assassinated by his own guards.
Caligula was resurrected and became part of the Triumvirate. Even though he doesn't appear until the Trials of Apollo series, it is stated that he and his organization helped fund Luke and Octavian, and therefore helped the villains of the previous series.
As the heroes try to stop the Triumvirate, he orders an attack on Camp Jupiter. He had planned on using Apollo to become the new sun god. Apollo stabbed himself to prevent this. As Apollo was being healed, Caligula defeated and killed Jason Grace, one of the main protagonists of the Heroes of Olympus series, and then went on to attack Camp Jupiter.
Caligula traveled with Commodus on his way to assault Camp Jupiter. During the assault, he and Commodus fought against Apollo and Frank Zhang. Frank fought Caligula in single combat and killed him.