septumsempra

It's first discovered by Harry in H.P.B but wasn't it used in the dueling club in the Chamber of Secrets movie?

One, this should be in the Ask Hermione Thread. Two, no. You must be thinking of Rictusempra, that is the closest sounding spell.

Yes, but this is a spoiler, seeing that it's in the HBP. Please read the Spoiler Annoucement and post in the appropiate threads.