Gigantes

The Gigantes are the children of Gaea and Tartarus. In The Lost Hero, Enceladus mentioned that each Olympian corresponded to at least one Gigantes.

History
The Gigantes weren't born until after the gods won the first Olympian War and took the throne from Kronos. While it was originally Gaea's idea for the gods to defeat her own children, the Titans, she was unhappy with the way the gods ruled and and gave birth to a new set of children, the Gigantes.

It was prophesied that only a god and demigod working together could defeat this new threat to Olympus, so Zeus called upon his own son Hercules to help win this war, called the Gigantomachy.

One by one, the Gigantes fell and all were killed except Aristaios, who was turned into dung beetle by Gaea so the giant might be safe from the wrath of Olympus. The only way to get rid of a Giant is for a god and demigod to work together against the Giant.

The Lost Hero
After the Second Olympian War, Gaea began to wake from her slumber and began to revive her children to once again fight the gods. Enceladus was one of the first to be brought back and was the giant that was born to defeat Athena. He kidnapped Piper McLean's father, Tristan McLean, in order to force Piper to lead Jason Grace and Leo Valdez to their deaths. However, this plan failed when Piper told her friends about her father. Jason and L eo agreed to help her and Jason fought Enceladus and defeated him with the help of a lightning bolt from Zeus, his father.

Gaea also managed to capture the goddess Hera, queen of the gods to use her power to revive Porphyrion, king of the giants and the one created to destroy Zeus, king of the gods. He is awakened, but isn't at his full strength. He runs away from the fight when Leo and Piper manage to set Hera free and she shows her true form to destroy the surrounding monsters.

The Son of Neptune
Two more of Gaea's children have been revealed to have been revived. Polybotes was leading an army to Camp Jupiter, the Roman equivalent of Camp Half-Blood in order to destroy the demigod children. He was created to destroy Neptune and shows that he has the ability to turn water into poison. He fails in his task however, as Percy manages to defeat him with the help of Terminus, the Roman god of borders.

Gaea's other child is Alcyoneus, the bane of Pluto. He was revived in part because of Hazel Levesque, a child of Pluto. Hazel was forced to help Gaea bring her child back, but eventually sacrificed her life in order to prevent his rise for a time. He can't be killed in his home territory and makes his new home in Alaska, the largest state in America. He manages to capture Thanatos, the god of death, so that Gaea can revive her monsters almost instantly and bring more souls to the land of the living through the Doors of Death. He is defeated however when Arion, a mythical horse, helps Hazel and Frank Zhang pull Alcyoneus to Canada where they manage to kill him.

The Mark of Athena
The two giants are revealed as Otis and Ephialtes, twin giants who were born to destroy Dionysus/Bacchus. The twins are stationed underneath Rome preparing for the arrival of the Seven. They trapped Nico di Angelo in a giant bronze jar to lure them into a trap. They have allied themselves with Arachne who has been guarding the Athena Parthenos for centuries.

Percy Jackson, Piper McLean and Jason Grace, confront the twins in their underground lair. The twins activate their plan to destroy Rome in a glorious spectacle but the heroes intervene. The giants nearly kill Jason and Percy before Bacchus appears. He teleports them to a reconstructed version of the Colosseum where Jason and Percy face Otis and Ephialtes for the god's amusement. The two eventually incapacitate the giants and Bacchus finishes them off.

The House of Hades
When Hazel Levesque meets Hecate she mentions the name of her mortal enemy: Clytius. She tells Hazel that if she is able to master The Mist and make it to the Necromanteion she will join the demigods in the final battle. Later Leo dreams of being chased by Clytius and notices that he absorbs all of Leo's fire.

When the heroes reach the House of Hades Leo and Hazel are separated from the others. They find the Doors of Death where Clytius is guarding the mortal side of the Doors. Hazel engages Clytius in battle while Leo tends to Percy and Annabeth who recently escaped Tartarus. The rest of the heroes soon arrive along with Hecate and together they are able to destroy Clytius.

In Tartarus, Percy and Annabeth met Damasen, the bane of Ares. Damasen is introduced to Percy and Annabeth by Bob. He heals their wounds and provides them with food to continue their quest. Annabeth, realizing that they need Damasen's help in order to finish their quest, asks Damasen to join them, but he refuses since he cannot escape his curse.

Later when the trio reach the Doors of Death, Tartarus appears and nearly destroys them until Damasen arrives, riding his arch enemy the drakon. Damasen battles his father allowing Annabeth and Percy their chance to escape.

In The Heroes of Olympus Series

 * Porphyrion - King of the Gigantes, Bane of Zeus/Jupiter
 * Polybotes - Bane of Poseidon/Neptune
 * Alcyoneus - The Eldest Giant, Bane of Hades/Pluto
 * Enceladus - Bane of Athena/Minerva
 * Ephialtes and Otis - Banes of Dionysus/Bacchus
 * Damasen - Bane of Ares/Mars
 * Clytius - Bane of Hecate/Trivia

Other Gigantes

 * Aigaion
 * Agrios (killed by the Fates by getting clubbed to death)
 * Aloadai (killed by Artemis by trickery in the myth)
 * Argus Panoptes (killed by Hermes in the myth)
 * Eurytos (killed by Dionysus in the myth)
 * Hippolytos (killed by Hermes in the myth)
 * Leon (killed by Hercules)
 * Mimas (killed by Hephaestus with molten iron, or by Ares, depending on the version)
 * Pallas (killed by Athena in the myth)
 * Pelorus (killed by Ares in the myth)
 * Thoon (killed by the Fates)
 * Tityus (defeated by Apollo in the myth)
 * Typhoeus (killed by Dionysus in the myth)

Trivia

 * Eurymedon is called the King of the Gigantes by Homer, but not much is known about him.
 * In the some myths Otis and Ephialtes are children of Poseidon