The Second Demigod Civil War is the second known time in American history that two different groups of demigods, Greek and Roman, fought each other.
Immediately after the Second Olympian War, Gaea began waking, with plans to resurrect each of her Gigantes children. The goddess Hera saw this as a threat, and pulled Jason Grace and Percy Jackson out of their respected camps and placed them into the others, wiping their memories in the process. Jason quickly gained the trust of Camp Half-Blood when he led a quest to save Hera from imprisonment in the Wolf House.
Nearly six months later, Percy made his way to Camp Jupiter and embarked on a quest to save Thanatos. When he returned, he was proclaimed as praetor, however, quite a number of campers opposed him, mainly the Augur, Octavian. Leo Valdez flew the Argo II over Camp Jupiter to gather the six other demigods, along with him according to the Prophecy of Seven.
Battle at Camp Jupiter
While the crew of the Argo II dined in Camp Jupiter, Leo gave Octavian a tour of the ship floating above the forum. An eidolon was possessing Leo without his knowing about it, and caused Leo to fire on the Roman camp. The Romans were quickly stirred up into a frenzy, and the seven heroes of the prophecy barely escaped. However, the campers continued to chase the seven, sending eagles to track them.
Battle of Charleston
When the seven were in Charleston to look for a map, Roman commandos attacked and outnumbered the seven nearly three to one. Annabeth Chase ran into Reyna midway through the battle, who warned Annabeth that if the seven were to sail to the Ancient Lands, then the Romans would march on Camp Half-Blood in retribution instead. Annabeth did not comply to the threat, and sailed east to Europe and Rome, leaving the Romans behind.
March to Camp Half-Blood
Over the next days, the Roman troops made progress towards the Greek Camp, as Reyna had learned the location of the Camp from Annabeth Chase, who had told her the location during their negotiations. The last they were seen was at Central Park in New York City. While stationed in New York City, Rachel Elizabeth Dare and Grover Underwood meet at the top of the Empire State Building in order to discuss the possibility of a truce with Reyna and Octavian. Rachel hands Reyna the note that Annabeth Chase sent from Tartarus, asking that Reyna bring back the Athena Parthenos so that peace can come between the two camps. Octavian also reads the note and believes it to be a trap or some kind of trick the Greeks are playing.
Reyna at first thinks this is some kind of distraction as well because the battle with the Gigantes is being fought in Greece. However Grover explains that despite the fighting taking place in Greece, Gaea is the earth itself and when she awakens, she will be everywhere; not just in the ancient lands. Reyna considers this and agrees to fly on Scipio to meet with the crew of the Argo II and retrieve the statue; which leaves Octavian in command. She orders Octavian not to attack Camp Half-Blood until her return, but of course, Octavian begins planning an attack the second she leaves, as she was the only one holding Octavian back.
Reyna, Nico and Coach Hedge embark on a quest to return the Athena Parthenos to the Greeks in a desperate attempt to stop the war. Reyna is outraged that Octavian had declared himself Pontifex Maximus, which was equivalent to a Roman Emperor. Her dreams further inform her that Octavian has hired former legionnaires, banished from the Legion for criminal activities, which include the psychopath, Bryce Lawrence.
As the events progress, Frank and Hazel find out from the goddess Artemis about the incoming battle. Octavian has summoned monstrous mercenaries like Cyclopes, two-headed men, Cynocephali and Centaurs, oblivious to the fact that they would turn on him after the battle. Octavian has also commissioned the Cyclopes to build six onagers, designed to hurl projectiles of massive explosive power.
Reyna, Nico and Coach Hedge eventually reach Long Island Sound with the help of Pegasus, Blackjack and six other pegasi. While on a yacht, Michael Kahale, Dakota and Leila come to arrest them but are stopped by Tyson and Ella who arrive with the help of Rainbow the hippocampus. Dakota and Leila, who are really loyal to Reyna, change sides as they always planned and they head back to the Roman camp with Nico to gather their Cohorts and stand against Octavian while Nico tries to sabotage the onagers.
Before the battle can begin, Nico, Will Solace, Lou Ellen and Cecil sabotage the onagers, causing their shots to detonate each others when they are fired, but it nearly starts a war when it angers the Greeks. However, Will is able to use a piercing whistle to stun both sides long enough for Reyna to arrive and order a ceasefire by presenting the Athena Parthenos. The presentation of the Athena Parthenos heals the rift between Greek and Roman and the gods as well, but Gaea awakens at that moment and her monsters attack. With the rise of Gaea and the return of the Athena Parthenos the Greeks and Romans put aside their differences and stand together against Gaea who is destroyed by the efforts of Jason Grace, Piper McLean, Festus and Leo Valdez. Octavian also fires an onager shot at Gaea that possibly helps, but also results in his death.
Finally, Camp Half Blood and Camp Jupiter make peace with each other, and Jason introduces new programs to strengthen the friendship while taking up the title Pontifex Maximus himself to honor all the gods after a promise to Kymopoleia. The two sides are also now able to visit each other's camps at will.