Some people say that the spell was already in the book but I say that Phoebe wrote it, I was just wondering what everyone else had to say about it. And hear is my evidence as to why I think it was written by Phoebe,
1) Why would she be so cocky about it if she hadn't?
2) It says "Halliwell witches stand strong beside us," so unless it was made by Grams or Patty, It had to be made by her.
3) It calls upon Prue, Grams, and Patty, Prudence is a name that have been used in the past by Warren Witches, but there are no known Patricia's or Penelope's so it would have to be one of the Charmed One's
This is all I can think of on the spot, but I'm sure theirs more, so comment and share your opinions