Troll Cave Dungeon
Troll Caves are a type of dungeon found in the black forest biome of Valheim. These places are sometimes guarded by a normal troll. If you enter the dungeon you can find a log troll which will be much harder to kill.
Finding these dungeons is an excellent source of troll hides. If you were wanting to farm trolls, search the black forest until you find trolls or troll caves. Trolls are a gigant and somewhat dangerous enemy, so be careful.