Trolls are a type of dark creature. Most trolls are bulky and strong while others are small like Bubda. Some are intelligent and cunning like Nero but some are quite dumb like the mountain troll in the fourth book.
There are many types of troll. Some include the River Troll, Cliff Troll (like Nero), Mountain Troll, and Hermit Troll. (like Bubda)
Cliff trolls resemble huge lizards with scaly skin, spikes which run down their backs and webs between their claws and toes. Nero is the only cliff troll seen. It is unknown if all cliff trolls are as intelligent or as skilled in magic as Nero.
Huge, bulky and fat, mountain trolls are the largest trolls. They are carnivorous and spikes cover their arms and backs. The troll encountered was quite dumb and it is unknown if others have the same nature.
These trolls are amphibious and covered in scales with webs which are better equipped for swimming than the webbing between a cliff troll's toes and fingers. A number of trolls serve the Singing Sisters.
Grip of the Shadow Plague
Nero was bitten by corrupted dwarves but since trolls are already dark creatures, the shadow plague didn't have any effect on him.
Secrets of the Dragon Sanctuary
Nero teaches Seth about his new role as a shadow charmer and helps him with some of his new powers. Seth befriends the mountain troll in Grunhold in order to steal the Soul of Grunhold.