The Giant Army (also called the Gigantes Army) is an army of giants, mortals, and gods and goddesses, who take part in helping the giant Porphyrion and the earth goddess Gaea destroy Olympus in the Second Giant War in The Heroes of Olympus series.
During World War II, Gaea began to awaken from her slumber. While the gods were busy fighting themselves, she sought to destroy them in their weakened state. She began making plans to cause her children, the Gigantes, to rise and once again take down the gods. Her plans began with a demigod daughter of Pluto, Hazel Levesque. Using Hazel's mother, she caused Hazel to move to Alaska and took control of Hazel's mother at night. She then used Hazel's power over the riches in the earth to reform her first son, Alcyoneus. Hazel however sacrificed herself to stop his rise and Gaea went back to sleep. Alcyoneus rose again during the 80's, as Hazel's sacrifice had only delayed his rising. He began recruiting monsters to his cause, including Roman Cyclopes and Centaurs. The other Gigantes also began to rise in response to Alcyoneus' awakening.
Meanwhile, in the Underworld, Medea used her abilities to see into the future and saw that a son of Hephaestus named Leo Valdez would someday go against Gaea. She informed Gaea of this child and Gaea went to try and break Leo's spirit by killing his mother, Esperanza Valdez.
Shortly after the Second Titan War, Gaea made her move and her children managed to capture the god of death, Thanatos. With Thanatos no longer able to guard the Doors of Death, Gaea set free mortals from the Underworld that would help her defeat the gods, who had closed off Olympus in hopes of luring Gaea back to sleep. Thanatos' capture also made monster reform much faster, further strengthening her forces.
Khione, the goddess of snow that joined forces with Gaea managed to trick Hera into coming to the Wolf House. When Hera arrived, she was trapped by Gaea in a cage of earth and had her power drained so Porphyrion, the king of the Gigantes could rise. Hera used her powers to transfer Jason Grace to meet a group of Greek demigods who would take him to Camp Half-Blood. It is at the camp where Jason is given a quest to rescue Hera. Jason decides to bring his two new friends, Leo Valdez and Piper McLean, along for the quest.
The Giant Army kept close tabs on the trio, as the giant Enceladus had kidnapped Tristan McLean, Piper's father, in order to make sure she led Leo and Jason into a trap. The snow goddess Khione also keeps watch over them, and hits them with snow storms during most of their quest. The heroes also run into monsters that had pledged their allegiance to Gaea, including the werewolf Lycaon, the Gegeines (also known as Earthborn), and Venti. They also learn that evil mortals from mythology were rising from the Underworld, most notably Medea, who tried to charmspeak Leo and Jason into killing each other, and King Midas, who tried to turn the three demigods into gold.
After Piper, Leo, and Jason manage to free Piper's father, the main forced of the Giant Army move to the Wolf House in order to take it back from the Hunters of Artemis, who were being lead by Thalia Grace in defending it. The lycanthropes and Earthborn attack the Hunters, who were trying their best to hold them back. However, the monsters would simply reform once destroyed thanks to Gaea's influence. The tide of the battle becomes even worse once Khione arrives and freezes most of the Hunters, including Thalia.
Despite Leo managing to force Khione into a retreat, Porphyrion rises from his slumber and plans on destroying the son of Jupiter, Jason Grace. Leo and Piper, however, manage to free Hera, who revealed her divine form, which destroys all the monsters (except Porphyrion, who escaped) in the area. The Giant army - as a result - was defeated, but managed to accomplish its goal of awakening the king of the giants.
As Percy Jackson made his way to Camp Jupiter, he was attacked by two of the Gorgons, who blame him for Medusa's death. Despite having faded, Gaea managed to bring them back and prevents them from being killed as long as they touch the ground. They also inform Percy during their fights that a large army of monsters is heading south to destroy the camp. Percy however managed to kill them by ripping them apart in the waters of the Tiber River.
Later, Mars reveals that the god Thanatos had been captured so monsters will not die and this will eventually effect demigods and even mortals. Percy, Frank Zhang, and Hazel Levesque are given a quest to free him, but run into many forces of Gaea on the way to Alaska. They soon learned that Polybotes was leading an army of cyclopes and Centaurs south to face the forces of Camp Jupiter, something the leader of the camp, Reyna Avila Ramírez-Arellano, already knew.
When Percy, Hazel, and Frank find the Amazons (as the leader was Reyna's sister), they find that Gaea had called the first queen of the Amazons from the Underworld to cause a civil war between them. Otrera, the original queen, convinced almost half the Amazons to turn away from Queen Hylla and join Gaea in her attack on Camp Jupiter, while Hylla wanted nothing more than to help her sister. For leadership of the Amazons, Otrera challenged Hylla to a fight to the death, but because Otrera could not die and would come back if beaten, Hylla had no hope but to simply hold out until Thanatos was free.
Once escaping the home of the Amazons, the trio seek the aid of a blind seer named Phineas, who Gaea had revived to help her see what lies in the future. He is killed and sent to the Underworld after losing a bet with Percy and as a result of the bet, they learned of the location of Thanatos. When arriving at the Hubbard Glacier, they ran into the eldest Gigantes, Alcyoneus. While Frank worked to free Thanatos, Hazel held off Alcyoneus, while Percy was forced to fight an army of Ghouls made up of the old Fifth Cohort. They had pledged their intelligence to Gaea after their death and now existed as mere shades. Eventually, Percy defeated the shades by taking them over a cliff of ice while Frank and Hazel dragged Alcyoneus to Canada with the help of Arion, Hazel's horse. No longer in his own territory, Alcyoneus was easily killed by the combined might of the two demigods.
The trio soon arrived back at Camp Jupiter, where Polybotes was leading the charge. The army of Cyclopes and Centaurs appeared to have the edge until the arrival of the Amazons. At first, they appeared to be helping the Giant Army, but quickly changed course and wiped out most of their forces with the aid of the cohorts. Polybotes as well was defeated with the combined effort of Percy and the god Terminus.
Due to the battle with Polybotes, Camp Jupiter, and New Rome are undefended and weakened Terminus' strength. When the Argo II arrives, Terminus states he would have blasted the ship out of the sky if he was at full power. After an attack on the city, Annabeth Chase asks Reyna if they're being attacked by the giant army again which Reyna declines.
- Porphyrion - bane of Zeus/Jupiter
- Enceladus - bane of Athena/Minerva
- Polybotes - bane of Poseidon/Neptune
- Alcyoneus - bane of Hades/Pluto
- Otis - bane of Dionysus/Bacchus along with Ephialtes
- Ephialtes - bane of Dionysus/Bacchus along with Otis
- Clytius - bane of Hecate/Trivia
- Orion - bane of Apollo and Artemis/Diana
- Periboia - bane of Aphrodite/Venus
- Mimas - bane of Hephaestus/Vulcan
- Hippolytos - bane of Hermes/Mercury
- Thoon - bane of the Fates
- Medea: a skilled Charmspeaker and sorceress.
- King Midas: able to turn anything he touches to gold.
- Lityerses: the son of Midas and a skilled combatant.
- Phineas: a blind man with the ability to see the future.
- Otrera: the original queen of the Amazons.
- Sciron: a bandit, son of Poseidon.
- Pasiphaë: a immortal sorceress.
Gods & Goddesses
- Khione: Goddess of snow. She actively worked to split the gods up from their demigod children by getting Zeus to close Olympus and capturing Hera.
- Aeolus: Predicted, since he sent the storm spirits to fight Jason Grace, Leo Valdez, Piper McLean, and the Hunters from saving Hera (tricked by Khione).
- Phorcys: Attacked the seven in Atlanta for the reward.
- Keto: Sent monsters to attack the Argo II.
- Boreas: God of Winter and the North Wind. Joined with sons, Zethes and Calais after Khione convinced them all.
- Nyx: Goddess of the Night.
- Tartarus: Darkest, deepest pit of the Underworld, second husband of Gaea, and father of Giants.