specifically, prior to the Flood of Light, the former Kingdom of Voeburt's court mage was dabbling in voidsent summoning as a way of destablizing the kingdom so he could seize the throne, he was the one responsible for spreading the 'sickness' around the kingdom, turning innocents into voidsent demons while at the same time pretending to be helping Branden and the royal family find the cause of the problem.
The court mage's duplicity eventually is revealed, but at the cost of Voeburt's Princess Saludia being turned into a Voidsent demon herself, requiring Branden along with Ardbert and the rest of his party to put her out of her misery (after dealing with that damn court mage of course).
There are still some remaining voidsent from this incident found in places around Il Mheg, but the Flood of Light and the fae mostly drove them into whatever shadows they could find to the point they were virtually forgotten, the Sineaters naturally becoming a much more greater problem.