The Witch Queen was the disguise of Lady Azarni.
She used the disguise to terrify the baron's food festival and scare him and everyone else away from the castle. After a long battle against Shaggy and Scooby, The Witch Queen was defeated, however, Mystery Inc. didn't get the chance to unmask the monster as the castle collapsed soon after the fight. In the aftermath, the gang revealed they believed Lady Azarni was responsible, since all the clues point to her. Although the baron denied he knew her. By the end, the witch queen was seen flying away, implying it was a real monster.