The Ultimate Battle

In 2006, The Charmed Ones went up against Billie and Christy Jenkins in order to get back Piper's husband back, Leo. A couple of weeks before that, they summoned the Angels of Destiny to help them stop the Angel of Death from taking Leo away. Destiny made a deal with the sisters, she'll stop Leo from getting killed if that participate in one great battle. However, to them they didn't find out that the evil they had to face was Billie, a young witch who they took into their care after they faked their deaths, and her sister Christy an evil-witch who was trained by The Triad to kill The Charmed Ones.

Escape
After The Magical Community abandoned them, they were forced to hide out in the underworld after Billie and Christy them in their own house. They took refugee in Phoebe's Condo a day later. As a last resort, they were forced to summon The Hollow to kill Billie and Christy once and for all. Although unknown to them Billie and Christy was summoning it at the same exact time.

Summoning The Hollow
Bille and Christy

After countless draw's with The Charmed Ones, The Triad comes to a decision for them to summon the Hollow, to kill the sisters. Summoning the Hollow takes a powerful force (as seen in Season 4 when The Source of All Evil summoned it) Although the Charmed One's were able to summon it themselves, The Jenkins sisters had to get the help of Wyatt Halliwell, Piper and Leo's oldest son to help. Dumain, a Demon who was picked by The Triad to help Billie and Christy kill the Charmed ones, shimmered over to their grandfather's apartment where Wyatt and Chris Halliwell were staying and took Wyatt back with him. Billie and Christy hold hands with the young boy and start to chant the spell to summon The Hollow. Once acquired, They stole Wyatt's powers so they'll be evenly matched with The Charmed Ones. As that the Hollow feeds on power they were about to steal Dumain's powers, but he shimmered out when Christy shot fire from her eyes.

The Final Battle
After killing The Triad, The Charmed Ones, orb to the manor. They turned around to see The Jenkin Sister's coming down the stairs. Both set of sisters are surprised that neither of them have potions. The Battle then starts to begin, Billie and Christy throw fire, whilist The Charmed Ones reply with lightning. The power slowly starts to trash the Manor, hitting many objects like Lamps and the grandfather clock. The power is so strong that it blows the Manor up. Killing Phoebe, Paige and Christy.

Aftermath
When Piper finds Phoebe dead, The Angel of Destiny returns Leo. Destiny tells Piper that the battle wasn't meant to end this way. She suddenly hears a moan, thinking it's Paige, she follows the sound, she realizes it's actually Billie and begins to beat her up, angry at her for killing her sisters. Leo tries to get Piper to stop beating up Billie, when he eventually does, she runs off and Piper tries to kill her using her Molecular Combustion power, but doesn't succeed. Leo tells Piper that they have to go. They take refugee in Phoebe's Condo