The Seer, who knows the Avatars' plans, decided to cooperate with them and the Charmed Ones: she will help them finding powerful demons if they make her human. Phoebe gets to be her friend, but the three sisters must protect her from a deadly danger: Zankou. Written by
Did You Know?
(at around 9 mins) Brian Krause (Leo) presumably misspeaks by saying "the demons don't want her working with us". He was most likely supposed to say "the Elders don't want her working with us", since the conversation is about how the Elders want Leo to protect the seer. See more
You know, if you want us to make a deal with the Elders to make you human, I think you need to give us something.
Wait. Back up. Did you say human?
I know, I know. I wouldn't actually have a soul, but I could live with that.