Gaea and Ouranos (the Earth and Sky) had the twelve Titans. When Gaea also gave birth to the Cyclopes, Ouranos threw them into Tartarus on account of their hideous complexion and one eyeball. An enraged Gaea gave Kronos, the eldest Titan, a scythe and he chopped up his father into little pieces so he could be a better king of the universe. Then Gaea fell into a deep slumber, and rested.
Gaea is attempting to rise and the Greek and Roman demigods have to work together to stop her. According to Gaea, she needs two powerful demigods, a male and a female, to be sacrificed. It is the only way for her to fully rise.
The earth goddess is introduced to the series as the antagonist after the Titan War. The "second chapter." She is first seen tormenting Leo Valdez as a child by coming into his house, not yet fully awake, and forcibly and tactically making him burn the house down with his mother inside. She is also referred to often as the earth mother. She is mentioned various times throughout the book yet never takes a physical form except for a few rare occasions.
- Face in the porta potty sludge
- Cage holding Juno captive
- Lady who scared Leo as a child in his burning house
The threat is not revealed until farther into the book. Gaea is called on indirectly when Gwendolyn is stabbed in the War Games but comes back through The Doors of Death easily and returns to life. Gaea has found a way to control who lives and who doesn't. When monster is killed they could return easily. It is so found she has captured the death god Thantos by resurrecting her son Alyconeus from Alaska. She also brings forth an army of monsters against Camp Jupiter which ultimately fails but is merely a small victory.
Gaea is seen numerous times in this book as well. The seven demigods unite together for the first time to conquer Gaea. She does not take physical shape however and uses her children the Giants and eildolins at her disposal to cause mischief between the Romans and Greeks and create problems and distractions for the demigods.
Gaea is mentioned but again, doesn't take physical form. Her demons and monsters guard the Doors of Death along with her powerful allies who all obey the Earth Mother.
SPOILERS FOR THE BLOOD OF OLYMPUS! READ AT YOUR OWN RISK
Gaea rises at the end of the book, in the final battle. She takes the mortal shape and as she is waking, she causes many demigods to sink into the ground. However, Leo scoops her up with Festus, carries her into the sky where she is weakest and with the help of Jason and Piper and Octavian they destroy her