The name "Fairy Storm" comes from the rare and dangerous Magic storms brought about by an abundance of Fairies.
In the world of the valley, Magic is usually a constant trickle. But occasionally for reasons unknown an area will suddenly swarm with Fairies and thus get an abundance of Magic.
People who survive Fairy Storms often describe them as nearly blinding from the cumulative glimmers from the swarms of Fairies.
Fairy Storms vary in intensity but can be extremely dangerous as tiny natural spells and effects in the area can suddenly surge and cause random and often dangerous effects. Enchanted items can shatter under the onslaught of power. Scrolls smolder and turn to ash. Even un-enchanted items can develop magical effects. Natural un-enchanted gems can either develop powerful abilities or be drained of their luster. Mages have been killed as the power fries their brains.
Fairy Storms can last from hours to days to months. Regions have been cut off from the outside world by walls of wild magic.
Luckily Fairy Storms are rare for any given place. Most Fairy Storms are tiny and affect only a small area or even just a single forest clearing.
In most out of the way little towns, Fairy Storms are little more than myth.