Ultimate Power

The Charmed Ones had confronted many forces of evil since they came into their powers but one of the last ones was so formidable that they almost died trying to defeat it.

Billie and Christy
At the beginning of the eighth season, the Charmed Ones met Billie, a young witch and Paige's new charge. Billie had recently come into her powers and was advancing very quickly. At first, she was presented with the power of Telekinesis but later on, she developed Projection, an extremely rare and potent power that allowed her to manipulate reality itself. Billie worked with the sisters to learn more about magic and, after years of searching, found her long lost sister, Christy, who was also an exceptionally powerful witch with the rare abilities of Telepathy and Pyrokinesis. However, her sense of good and evil had been severely warped by the Triad and she now believed the Triad was on the side of good while the Charmed Ones were evil. During the process of getting accustomed to the world again, a Triad member revealed to Christy that Billie was the Ultimate Power and that she must save Billie by destroying the Charmed ones. Soon, Christy convinced her sister that the Charmed Ones were evil and that it was their destiny to stop them.

Working together as sister witches, they combined their magic to become unspeakably powerful. For instance, they were able to destroy a Noxon Demon, who was supposed to be impossible to vanquish. Their combined magic was enough to burn the demon up from the inside and blow him up. Soon their destined strength and magic not only rivaled the Power of Three but also forced the Charmed Ones to resort to the utterly desperate measure of summoning The Hollow to fight back. However, Billie soon realized that Christy was brainwashed by the Triad and returned to the side of the Charmed Ones. Christy went back in time to warn the Triad but so did Billie herself and the Charmed Ones using her projection power. This helped the Charmed Ones destroy the Triad once and for all. She then begged Christy one more time to turn away from evil. Christy launched a huge fireball at Billie but Billie deflected it back, inadvertently killing her sister in self-defense.