The Voiding was the event that changed the course of the war with the Southern Hordes. Magaras Thul a powerful blood mage created a spell to destroy the entire Hobgoblin army. Though it was successful in destroying the army, the cost was high. The Empire’s population was decimated to fuel the spell raising many of these people as angry spirits that rampaged across the Empire.
It was this one act that called for the Emperor to ban all blood magic and users of blood magic as athema and beyond redemption in the eyes of the Emperor.
The ongoing ramifications of the voiding are that certain areas of the prime material plane have become intertwined with The Void. This allows for denizens of that plane sometime to pass over into the material. Most of these have now been contained by Order of Faith but once in a while another one has been found.