Walpurgisnacht was the annual celebration of the downfall of Elias Bram and the night Solas fell. Demons are most powerful at this time of the year. It became a bloodbath after Astaroth was fooled in front of all his vassals and dukes.
In the Fiend and the Forge, Walpurgisnacht is introduced as the celebration of the downfall Solas. David attempted to poison Astaroth, expecting it to demonstrate his 'Great Power' by crushing David and Max, and any hope of Human resistance. Astaroth anticipated that David will feed him Blood Petals. What he did not know was that the 5th vial contained something different. When Astaroth(and he other demons) realized that Astaroth was tricked, they immediately rebelled, the strongest going after the Book of Thoth. They did not know Astaroth still was more powerful, despite the fact he was weakened. Max massacred most of the demons, during a effort by Prusias to attack Astaroth. Soon, he had killed Lord Vyndra, and a lot of other demons. He nearly took Prusias`s head. In the end, Astaroth defeated most of them using the Book of Thoth, but Elias Bram lead a narrow escape via teleport. Walpurgisnacht will be forever remembered as a turning point in The Great War, marking the fall of Astaroth and return of Elias Bram.