Native to northern Europe, trolls are a tough, headstrong, tribal species. Most of them can be found in Norway, and it's said that ancient viking warriors would sometimes hunt them for sport in order to test their martial prowess. Trolls are unsophisticated but brutal warrior giants that love to fight. They don't have much in the way of culture or intellect, but there are few better monsters for wading into a battlefield. The stupidity of trolls is a very common stereotype though that stereotype exists for a reason.
The trolls' other claim to fame and a much more positive one - is their ability to regenerate. A troll can heal a wound in a matter of moments, even if the wound causes critical damage to major organs. Lost limbs regrow and reach full strength within a day, and even a troll that has been, for all intents and purposes, killed will usually recover, sometimes before their opponent has even caught their breath. There are even records of trolls being hacked to pieces, only to regrow entire bodies from the stump of their necks.
If one wants to hurt a troll permanently, or at least permanently enough to win, one has to either inflict a truly massive amount of damage at once, or keep the wound from healing naturally. Fire and acid are the most popular methods, but ice and poison will also impede the growth of healthy flesh. Alternatively, you can just stick a weapon in them and leave it in to keep the wound from closing, although being unarmed against an angry troll is probably a worse situation.