It started at the end of The Sea of Monsters and progressed until The Last Olympian. It was caused by the Titans rise in power, culminating in the resurrection of Kronos. Kronos convinced several of his old allies that had been in hiding and several minor gods to join his cause and wage war on the Olympians.
The Olympians eventually won the war, but the damage caused by it caused Gaea to reawaken from her long sleep and several more monsters. In The Lost Hero, it was stated that Zeus thought that if the gods cut themselves off from their demigod children and the world, Gaea would fall back asleep, but apparently it didn't work, since Gaea and the Gigantes were awoken as results of the Great Stirring.
Awakened Immortals and Monsters
- The Ophiotaurus
- The Clazmonian Sow
- Tammi, Kelli, and another unnamed Empousai
- Nemean Lion
- Erymanthian Boar
- Cancer (Karkinos)
- The awakening of Gaea, along with the rebirth of the Gigantes came as a result of the events of the Percy Jackson and the Olympians series, but was not part of the Great Stirring.