This was the moment Agatha took control over Amnesty's body, which went wrong, resulting in a massive outburst of magic energy. When Amnesty regained control, the king and queen were both gone, and Drac had mutated into a half-man half-snake creature.
Conquest of Darkness
In the story "Conquest of Darkness" Drac returned to Silvast in hopes of taking revenge on Amnesty. His goal was to take control of Silvast and make Amnesty pay for his mutations at the same time. He was defeated by Arbiter and Amnesty before he managed to fullfill any of his goals.