Diocletian

Diocletian (Latin: Gaius Aurelius Valerius Diocletianus Augustus), also known as Diocles, was the last pagan Roman Emperor. He was the son of Jupiter, making him the last demigod to rule Rome. During his career, Diocletian became responsible for the deaths of several Christians in his attempted persecution of Christianity. Weakened by illness, Diocletian became the only Emperor to voluntarily abdicate the throne. He lived out his retirement in his palace on the Dalmatian coast (in modern-day Split).

After his death, Diocletian's ghost was said to haunt his the city of Split. His sceptor held the power to reawaken dead Roman legionaires. It could only be wielded by a child of Hades, and the spirits would only follow an officer of Rome. The sceptor became a turning point in the second Giant War, when it was recovered by Jason Grace and Nico di Angelo.

Before the Series
While attending St. Agnes Academy for Colored Children and Indians, Hazel Levesque was taught that Diocletian was a huge villain, responsible for the deaths of several Christians.

On the other hand, decades later, Diocletian was admired by the Roman praetors Jason Grace and Reyna Avila Ramírez-Arellano. The praetors always imagined visiting Diocletian's Palace in Croatia, but they knew that that was impossible. They saw him as one of Rome's greatest rulers.

The House of Hades
After receiving a message from Annabeth Chase about the Athena Parthenos, Reyna resigned herself to making the trip to Greece, in hopes of preventing a war. She would head for Diocletian's Palace, believeing that if the Seven were in Europe, that was the one place Jason would stop. Fortunately, Jason witnessed all of this in a dream. He made plans with the group to head for Diocletian's Palace.

After the sceptor was retrieved by Jason and Greek demigod Nico di Angelo, the latter wielded it in the battle inside the House of Hades. Jason was unable to lead the spirits, as he had just recently defected to Camp Half-Blood. He stepped down as praetor and promoted centurion Frank Zhang to praetor. Frank led the legion of spirits to victory against Gaea's forces, until a combination of demigods, Titans, and Gigantes worked to seal the Doors of Death to prevent anymore enemies from escaping from the depths of Tartarus.

Trivia

 * Diocletian's birthdate is believed to be December 22, 244. This would mean that he shares his birthday with his half-sister Thalia Grace, daughter of Zeus.