What kind of damage does the Crepuscular Ray ability deal?

Since the ability uses the arrow storm spell as its base, it seems like it should use spell damage as its damage type. However, when the ability is being used, all it does is shoot a bunch of regular arrows at enemies, which implies that its damage type is Main Attack. Does anyone know which is correct, I want to make a busted main attack damage build.

 

All damage in Ability Tree is Spell Damage unless it explicitly states otherwise (such as Twain's Arc)

 

Thanks for the tip!