Creature of Havoc is probably the toughest Fighting Fantasy gamebook in the series. (Although Crypt of the Sorcerer is also unfairly tough in a different sort of way.) It also features the most interesting premise, as you start out as a monster with no idea of who you are, or the strange evil which is rising up around the corner.
In terms of story and writing, I think this is easily the strongest of all in Fighting Fantasy world, as you slowly struggle to discover your identity and your sense of purpose. There are plenty of ways to get sidetracked along the way, and inadvertently end up working for the enemy. Tha game's backstory makes for pretty good reading, and it is also interesting to see it getting fleshed out in the world later on.
The game's signature feature is the garbled language which other characters speak. Over time, you might find a way to translate it, but doing so is incredibly tedious and not fun at all. You will need to do the translation if you are to complete the game, so you might want to google a list or something, beacause doing the busywork yourself just isn't fun.
I think this could have been a much better novel than game, and as much as I like to complain about the writing in the Fighting Fantasy series, I think this one is a true masterpiece of story and lore. Shame it is so difficult to complete. Free read is pretty fun, however.