In the fourth book of the series, there is also a fight against Tartarus, which, in Greek mythology, was the darkest and deepest point of the Underworld.

The Heroes of Olympus is centered around a prophecy introduced in The Last Olympian that predicted seven demigods would unite to protect the world from an awakening new enemy, the Earth goddess Gaea.

Some other important characters are Nico di Angelo, son of Hades; Reyna Avila Ramirez-Arellano, Praetor of Camp Jupiter and daughter of Bellona; and Gleeson Hedge, a satyr.

The novel begins by introducing Jason Grace, Piper McLean, and Leo Valdez, three newly discovered demigods who travel to Camp Half-Blood in response to a prophecy.

Jason, who has amnesia, also begins to remember pieces of his past throughout the book —most importantly, he comes from a Roman camp for demigods.

The novel opens with Percy Jackson, struck with amnesia, discovering Camp Jupiter and meeting several Roman demigods.

After successfully completing their mission, the group returns to Camp Jupiter to defend it from yet another giant, Polybotes, and his army.

After the Argo II unintentionally fires on Camp Jupiter after Leo Valdez is possessed to attack it, the seven demigods of the "Prophecy of Seven" — Percy Jackson, Annabeth Chase, Leo Valdez, Piper McLean, Jason Grace, Frank Zhang, and Hazel Levesque — rush to escape the angered Roman campers.

Hazel and Leo go out to try to find Celestial bronze and lime for calcium carbonate when they encounter Nemesis, the universal (meaning Roman and Greek) goddess of revenge.

In Tartarus, Percy and Annabeth travel towards the "Doors of Death", the only pathway back to the mortal world, with help from Bob the Titan, hoping to escape and simultaneously stop more monsters from getting through.

They also meet with Reyna Avila Ramírez-Arellano, a Roman demigod; she, Nico, and Coach Hedge leave with the Athena Parthenos from Epirus, Greece, and Shadow Travel to Camp Half-Blood.

The four stories include: Octavian Full Name: Octavian Affiliation: Camp Jupiter (Roman demigod) Godly Parent: Apollo (Legacy) Role: Augur (prophet) of Camp Jupiter Weapon: None directly, but he manipulates power through prophecy and politics Notable Traits: Skilled in reading prophecies (by disemboweling stuffed animals) Extremely ambitious, power-hungry, and manipulative Weak in direct combat but strong in political strategy Holds deep resentment toward Greek demigods Octavian is cunning, persuasive, and dangerously ambitious.

Key Actions in The Heroes of Olympus: Influences the Roman Senate to go to war against the Greeks Tries to undermine Jason, Reyna, and Percy’s leadership Ultimately meets his downfall through his own overconfidence Fate: His arrogance leads him to a tragic end when he miscalculates a weapon launch and is killed by his own attack.

Although initially uncertain how fans would react, Riordan later found that they enjoyed the new format, as it allowed them to learn more about each character.

[14] Riordan says that "It was my way of letting them revisit that world in a fresh twist, but also to catch up with Percy and Annabeth and the rest of the gang from the first series".