Skeleton Stonehenge Dungeon (Burial Grounds)
The Stonehenge dungeon in Valheim can be found in the black forest biome. Skeletons will guard this place which hit for around 15 to 30 damage with each shot when you are unarmored. You can enter this dungeon like an instance and raid it.
If you want to see one for yourself, check this Valheim Dungeon Seed.
Burial chambers dungeons are also where you can find surtling cores in the early game.