Location: Many fantasy worlds, under bridges
Threat Assessment: 2-4. Depends on the world. Trolls are generally extremely large. Extremely strong, and extremely aggressive.
Limitations: They are also as a rule extremely thick. Trolls in some worlds will turn to stone. Consult your mystic scrolls before exposing your troll to light.
Personal Note: My favorite trolls are those on the Disc World. They have become to some extent civilized though they still favor physical solutions to problems they encounter. Disc World trolls are silicon lifeforms and eat minerals. They have diamond for teeth which probably led to their inevitable problems with dwarves who probably acted as impromptu dentists.