Der Ketzer (The Heretic) is a German generic fantasy adventure and sourcebook.
The book describes the city of Gâdiz di Héo in detail, which is lightly based on real-world Venice. The sourcebook part is 84 pages long.
The 28-page adventure throws the characters right into a coup of the Sun Warriors that seize the power in Gâdiz di Héo. There are several possible ways to solve the adventure: in a diplomatic way, with open warfare or with an undercover operation.