When the god of arcane magic, Uthirian, committed suicide, his spellstorm ripped through his followers battling in Northire. The arcane practitioners lost their magic, and were infected with insanity and rage. It also slightly changed their physical bodies, making the typical Northiran ethnic build more solid and taller. When they calmed, their former enemies found that their memories were gone, but that their former arcane potential had left a void filled naturally by psionics.
Maenad culture is incredibly similar to old Northiran with a few differences. Maenad's can appear stilted and formal to some other races, and many consider them a little too reserved. This is a cultural adaptation to the raging emotions inside them, as they speak and act in such away to make their intentions perfectly clear and logical to prevent causing offense.
Maenad's have spread out, but are still more common in areas formerly held by Northire. They are also common in Maryx, secretly holding the reigns through a network of psionic thrallherds controlling from behind the scenes.
Wilders are especially common amongst Maenad psionic characters, as the emotional release is appealing to many of them.