Once you have defeated the Elder Boss is defeated, collect Elder Trophy and Swamp Key. These two items are going to help in further progression in the game and in defeating the next upcoming valhiem boss. Swamp Key is used to unlock “Sunken Crypts” in the upcoming level.
Bonemass is the third boss you will face in Valheim and it is located in Swamp Biome. It is a Valheim third boss must be summoned at a Bonemass Mystical Altar. To find the correct location of the Bonemass Mystical Altar you need to interact with a red glowing runestone inside a sunken crypt. You start getting iron in the swamp, which helps with the combat
This Bonemass boss fight guide explains to you about Bonemass power, Bone mass weakness, Bonemass Valheim offering, and trophy.