Neverwinter Nights was the same, spell effects (damage and so on) was one call from a script, the visual effect was another. If you wanted an ice-ball you would

Neverwinter Nights was the same, spell effects (damage and so on) was one call from a script, the visual effect was another. If you wanted an ice-ball you would steal ice visuals from some other cold spell, then apply cold damage instead and voila, an "iceball" instead of fireball. Most spells were very easy to mod like this, just because it was done already with a few lines of scripting in the game to begin with.