Zagor, the warlock of Firetop Mountain is a 10th level Sorcerer. The adventure was created for a fairly low-level party of 1-4 4th level characters, so having him a higher level would make the adventure very difficult to complete. There are suggestion in the appendix for levelling Zagor if you intend to use the adventure for a higher level party.
I actually changed him to a wizard..just seems right for him IMO, plus he has spellbooks- which are useless to a sorceror..:) Hope Jamie doesn't mind me tinkering with his module.