My father used to work at the publisher Ploegsma, so I got a lot of YA books in my youth. This book, together with another title by Van Kerkwijk 'Griezelgolf' remain my favorite books.
The story is horrifying - Waldemar and his two children, Meindert and Siebold, flee from their city in Germany after Waldemar's wife, Hilda, is burned at the stake for witchcraft. Count van Everdingen, Hilda's last client, kidnaps her body from the stake, but her soul has already been taken by Hilda's old teacher, Samant the White. Since Van Everdingen cannot exact revenge on Hilda, he pursues Waldemar, Meindert and Siebold, all the way to Holland. When Van Everdingen catches up with them, he manages to get Waldemar and Meindert to drink from a strange vial containing 'the blood of the devil', which burns their souls away and turns them into sociopaths like Van Everdingen.
Siebold manages to escape that fate, but is drafted into the East India Company, while Waldemar is caught by Samant who attempts to exorcise the demon. Meindert and Van Everdingen join a crew of pirates. From there on, Siebold repeatedly crosses paths with his changed brother Meindert, who has only two ambitions left: revenge on Van Everdingen and becoming a feared pirate.