Boom effect
From Alter Aeon Wiki
The boom effect occurs when a ball of lightning hits a mob and is struck by a lightning bolt. Booms do far more damage than either spell on its own. It requires at least 2 players to successfully boom a mob. A single player, even with haste, cannot.
Note that a solo mage with a lightning elemental will be able to trigger this effect, by casting counter spells to those of the elemental.
