Projectile Thunder Field is a static projectile 呪文 that combines offensive & defensive aspects. It creates a sparking, magical field at the casting location (melee, unless modified). Any projectile that comes in contact with it, either from the outside or inside, will be transformed into a 稲妻ボルト directed away from it.
It's a powerful defensive barrier that can be very hard for enemies to penetrate. You'll still be vulnerable to melee attacks, though, as well as everything else that is not a projectile.