Well, it was Eragon's first spell, and it will probably be the one that kills Galbatorix.
I have my doubts on whether or not there will even be a second movie. Whoever wrote the script obviously just picked up the main points from a friend, rather than read the book himself, because he barely followed the book and worse, killed off the Razac! They are big time important characters of the second book. That, and other changes pretty much cripple any chance of a second movie. (Like, no Solembum (the werecat)? What about Elva (the baby Eragon blesses, as she comes to call herself Elva in the next book. Eragon's spell makes her grow quickly.)?
Unless these two characters are introduced in the second movie, and the Razac really aren't dead, (as if turning into dead locusts and dirt is their way of escaping death, or there are two others) there isn't much of a chance for a sequal.
Not to say the Eragon movie was bad, I really liked it, it just takes too many liberties and drops a lot of the story. Not much chance for another movie.