After Rhea's husband, chief of the Titans Cronus, overthrew his father Uranus, Uranus issued a prophecy. This stated that just as Cronus had rebelled against his parents, so would Cronus's own children in turn rise against him. Growing increasingly paranoid at the thought of this, Cronus proceeded to swallow each of Rhea's children (Hestia, Hera, Demeter, Hades and Poseidon) as they were born. Deeply upset by this, Rhea resolved to put an end to Cronus' rule. To this effect, she gave the last of her children, the future King of the Gods Zeus, to her followers to hide away, and handed Cronus a boulder wrapped in blankets which the Titan devoured, believing it to be his son.
Many years later, once the boy had become an adult, Rhea had Zeus give Cronus a potion which caused him to vomit up the somehow-unharmed Olympians, and so began the Titanomachy, the ten year battle for rulership of the cosmos waged between the Olympians and their allies and the Titans.
Rhea's fate following the defeat of the Titans is not recorded, although it may be assumed that as she aided the Olympians she was not cast into the Abyss of Tartarus along with her kin.