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Lightning Bolt is the first of the upper level, damaging lightning spells. Until this point in the tree, lightning damage required the use of the weaker Static Blast or the touch spell Shocking Grasp.
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 two players to successfully cast. A single player, even under the effects of haste, cannot.
Note that a solo mage with a Lightning Elemental will be able to trigger this effect, by casting the appropriate spell to complement those of the elemental.