Out of the void Chaos created Gaia, goddess of the earth, and Uranus, god of the sky. The gods of sky and earth had numerous children but Uranus confined them to Tartarus, except for twelve who became known as the Titans. Of these, Cronus was the youngest and the most rebellious in temperament. Prompted by his mother, Cronus overthrew Uranus and imposed his rule on the divine and mortal worlds.
At first it was a golden age but Cronus became more tyrannical and paranoid. A prophecy of his children rebelling as he did to his father haunted Cronus, so he ate each child soon after birth. Appalled by this, his wife Rhea hid the youngest boy, Zeus, from Cronus and in time Zeus went to war with his father. With help from those oppressed by Cronus the Olympians came to power and the Titans were defeated.
Zeus declared himself King of the Gods and stripped the Titans of their realms of power. Some of the realms were given as rewards to Olympians for their service in the war but others fell into abeyance. To avoid conflict among the Olympians, Cronus' realms were drawn as lots by Zeus, Hades and Poseidon.
The new king handed harsh sentences to the Titans. Those who fought in the war were imprisoned in Tartarus while those who remained neutral or surrendered faced exile. The Titans have grievances against the Olympic court and against individual Olympians. However, the Titans are not a unified force. Each has their own goals, friendships and enmities with other Titans. The biggest faction are those in Tartarus who still follow Cronus. Their goal is to win freedom and restart the war. Other Titans' goals are smaller in scale, such as confining their ire to a single Olympian or regaining power over a particular realm.
Worship of the Titans is illegal and considered blasphemous wherever the Olympians gods rule. Yet the gods and their priests cannot be everywhere and small Titanic cults exist across Greece. They have meeting places and temples, hidden inside a city's walls or far into the countryside away from passers by. The cults draw to them men and women who feel the Olympian gods have failed them in some way.