The Great Fireball (GFB) rune shoots a fire attack that damages all creatures (except those immune to fire), and players (unless in a non-PvPgameworld), within a large circular area centered around the square where the GFB is targeted. The damage area will be a total of 37 squares, as shown in the picture, unless some of the squares are obstructed. The player using the GFB rune is never affected by it.
This rune is really useful to get rid of big groups of monsters when the character gets surrounded or when you need to clear a room. As the Magic Level raises, the rune will become more effective, to the point that sometimes the FB rune will be enough, which allows you to save the GFB for harder situations.