Titans
Approximately 1000 BC, the Elders saw the Titans as a risk to the world and themselves. So they unleashed the powers of the Greek gods onto mortals, who were then able to use that power to imprison the Titans for millennia. Future Chris Halliwell travelled back to 2003 to warn the Charmed Ones of the impending danger of the Titans.
When the Titans were released, their ultimate goal was to seek revenge against the Elders and rule over the world. Their first step then was to turn two Whitelighters to stone, and steal their ability to orb, as its the only way to get "up there." After they did manage to orb up, they killed numerous Elders, with the survivors seeking refuge on Earth. To fight them, Leo Wyatt went up to request the Greek gods assistance. They later returned to find him, sensing someone was left up there. Eventually, they were defeated while on Earth, and the remaining Elders were able to return.
Piper Halliwell was granted the powers of the Earth, a kind of "mother nature." She could cause earthquakes, especially when angered. To destroy the Titans, Piper opened a chasm in the earth, in the conservatory, that swallowed the Titans.
The Titans were made up of:
Cronus: was a Titan in Charmed, he appeared in Season 5 and worked with a fellow Titan named Demitrius to get revenge on the elders. He could control fire.
In Greek mythology, Cronus succeeded his father Uranus as ruler of the universe. He fathered six of the twelve gods and goddesses in Greek mythology, including Zeus. He swallowed his children when they were born, with the exception of Zeus, so as to prevent them from overpowering him. Through a careful plan, Zeus was prevented from being swallowed by his cruel father, and was cared for and grew up to lead his other brothers and sisters to overthrow their father
Demitrius: was a titan who controlled electricity and created thunderstorms. He was Meta's lover.
Meta: was an ancient Titan, she worked with fellow Titans, Demitrius and Cronus, to get revenge on the Elders. She had the ability to turn people into stone by looking into their eyes and lighting fires when she is angry. She was Demitrius's lover. Cronus killed her in "Oh My Goddess!", Part 1, for exposing them to The Charmed Ones.
The Titans in standard western Greek mythology... season five in the two hour season finale "Oh My Goddess!"
Read more about this topic: List Of Magical Beings In Charmed