Arcane Missiles is Arcane's hard-hitting proc spell, filling a role similar to Pyroblast for Fire. It's available to cast when its buff procs from harmful spells. All such spells have a 30% chance to proc Arcane Missiles. In Mists of Pandaria, you can accumulate two stacks of Arcane Missiles charges, allowing for two consecutive casts.
Casting Arcane Missiles adds one Arcane Charge. Its damage is increased by charges; as the spell costs no mana, there is no cost increase.
Arcane Missiles does not benefit from the charge it adds. If you cast it while you have two charges, it will deal 144% damage, even though the third charge appears at the beginning of the cast.
The damage increases with Arcane Charges as follows: