Curses! I knew I should've argued harder with my friends to stay after the credits. Shit.

As per Jean, I don't think it's lifted straight from Willow. What it did appear to look like is imagery pulled right off of The Rage: Carrie 2, which, IIRC, preceded the BtVS Willow-turns-evil storyline.

Either way, it's obviously powerful if it keeps reappearing.