This is one of the better ways for Dervishes to keep spirit spamming Ritualists at bay, since you can damage the spirit, all nearby spirits, and probably the caster as well.
The skill's bonus damage activates for each summoned creature you hit.
This skill is amazingly effective against minions, which tend to stay clumped up together. Aside from their already low armor and health, the Holy Damage dealt to them is doubled. Hitting multiple minions will most likely wipe out an entire minion army, or at least damage them enough for the next strike to finish them off.