Ok I don't own anything from Buffy or Charmed. It takes place during both of the shows sixth seasons.

The Ultimate Enemy

"The Charmed Ones will be able to kill them," Ignis says. "How do you know?" her minion says. "Because they are the most powerful witches of all time. Plus they've have an endless group of allies," Igis says. "They may be able to do the one thing no else has been able to do before: kill Buffy Summers and her friends," Ignis says. "Now let's perform the spell,"

The sisters are all sleeping when they hear the sound of a crying child. They all get up and go into Wyatt's room. The sun is barely rising. He isn't awake. The crying doesn't stop. "Who's crying then," Paige asks. "Demons," Piper says. The crying grows louder and it comes from the attic. They all go to the attic and see a child on top of the book of shadows. "Is this a trap," Paige. "Well do you want to check," Phoebe says. "Let's just check already," Piper says. The baby then disappears. It appears again in front of them and then becomes a red mist. They charmed ones inhale the mist and their eyes become red. They look at the open book of shadows. The open page is about a group who call themselves the Scooby Gang. "The Scooby Gang, a group of supernatural humans who are chosen to end all of humanity," Phoebe says. "Wonder why the chose such a crappy name to call themselves," Paige says. "They must be killed by the charmed ones before the winter of mercy," Phoebe reads. "Look in the book about the winter of mercy," Piper tells Phoebe. "I am," Phoebe says while flipping through the pages. "The Winter of Mercy is the season where the underworld's savior arrives and unleashes all hell on Earth," Phoebe says. "We need to find them and kill them," Paige says. "But they are mortals," Phoebe tells her. "Well that doesn't make them innocents," Piper says.

They search for them and discover they live in Sunnydale, California. "Should we do it now or what," Paige says. "Let's make potions first," Piper says. They make the potions and orb to Sunnydale. They are outside Buffy's house and kick open the door. Buffy comes with a stake in hand. They enter the house and say, "We're the Charmed Ones and you're all out of luck." She comes after them and they throw the potions at her. An explosion goes off and the house explodes. Everyone is electrocuted instantly and dies. The Charmed Ones orb away before the explosion. Willow comes home from the grocery store and sees the house. She goes inside and sees everyone is dead. Willow eyes the turn red. She attempts to revive them, but Osiris the god appears in front of her. "They can't be revived because the potion that killed them is irreversible. They only people who could have made a potion like this are The Charmed Ones," he says. She telepathically forces him away. She goes to the Magic Box and absorbs all the dark magic within the books. Her eyes and hair turn black. She changes into dark clothing also. She then flies to San Francisco.

Phoebe is pouring cereal when she has a premonition. In it a woman with dark hair and eyes kill them brutally. "Piper! Paige!" she screams. They come instantly. "What is it," Piper says instantly. "I had a vision where a woman kills us," Phoebe says. "She had black eyes and hair," Phoebe says. Then Dark Willow comes flying in. She waves her hands and Phoebe goes crashing to a wall. Paige grabs Piper and Phoebe and they orb to the attic. "Magically lock the door," Piper says. Phoebe throws a blue liquid on the door and it absorbs it. Dark Willow goes up the stairs and throws a fire ball at the door. It bounces back at her. "Let me in!" Willow screams.

Inside the attic The Charmed Ones are summoning the most powerful magic source known, The Hollow. In the center of the earth, The Hollow is escaping its prison. The Hollow enters all three sisters. All of their eyes go black before going back to normal. They open the door and Willow rushes in. "Just because you let me in doesn't mean I'll have mercy on you," Willow tells them. "You'll be begging us for mercy," Piper says. "You killed my friends," she says. "You were the underworld's savior," Phoebe says. Then dark purple flames come from Willow's hands. Bright blue lightning comes from the charmed ones hands.

Who should win? And the demon in the beginning will be further explored.