Another book I read as a child: Chronicles of Kvähl by Dennis Jürgensen. The boy and girl in the picture were destined to be part in a magic potion that would allow a vampire demon to be immortal. Being that this was a children's book, the more serious subjects like the whole drinking blood-thing was toned down, most noticably by the fact that these four trolls (and their companions in another group) were dumber than a sack of rocks and sometimes more interested in beating each other up than catching Arnold and Katarina here.

Lots of swearing and random violence going so still one of my favourite books.