Bonemass is Valheim's third boss and can be summoned in the boggy Swamp biomes. Despite his name, Bonemass more closely resembles Oogie Boogie from Tim Burton's A Nightmare Before Christmas, and he's every bit as toxic, if not more.
Preparation And Summoning Bonemass
To reach the Swamp biomes, you will have to brave the open seas. Due to the severity of the voyage, you're going to want to build a boat more substantial than the raft you've probably been using thus far. Boats require bronze nails to construct, which aren't too difficult to fashion after you've developed a functional smithy complete with a smelter and forge.
Boss location runes can be found inside castle ruins or sunken crypts within the swamps. The crypts require the swamp key you obtained from killing the second boss, The Elder, in the Black Forest biome. If you're going to search the crypts, remember to bring a pickax because most of the doorways throughout the labyrinths are blocked by "muddy scrap piles," which yield iron scraps and withered bones.
Iron scraps are obviously useful, but you'll need ten of those withered bones to summon Bonemass. Withered bones can also be found ambiguously sitting on shelves and stored in chests within the crypts.
Recommended Gear
Bonemass has a blunt trauma weakness and strong resistance to both slashing and piercing, making it imperative to use weapons like war hammers or iron maces.
Unless you've managed to find Haldor, the trader, and have spent a good deal of time in the Swamp biomes already, you probably won't have the materials to craft an Iron Sledge. So the next best weapon will be a fully upgraded Iron Mace. Along with the mace, you should bring along a round shield for protection against the few attacks you can block.
Poison Resistance Mead is also a must for this battle. This brewable mead reduces poison damage by 75% and remains active in your system for ten minutes. You should have a cauldron by now, and a fermenter is easy enough to craft at this stage, using bronze, resin, and fine wood from birch and oak trees. After setting up your brewery, brewing mead is a simple task.
Most of Bonemass's attacks deal hefty poison debuffs as well and the minion blobs he can spawn also love to spew poison clouds. In short, you won't make it through this battle without taking some poison damage, and it will be near impossible to make it out alive without neutralizing the effects somehow.
You'll want to upgrade your armor as much as possible as well, due to the fact you can't rely on ranged tactics and will need to get up close and personal with the giant glowing green mass.
Fighting Tactics
Bonemass uses a set of three attacks. His poison barf causes a large AoE you can recognize by a large green cloud that forms as the bile spills out of his mouth.
Next is his melee arm swing, which is capable of taking down trees. Be alert and stay out of the radius of the falling foliage, but you can also parry the blow. This is the best time to get a few hits in right after he swings, so try to parry as often as possible. If you didn't bring a shield and opted for a two-handed club instead, just be sure to rush back in range after steering clear of any falling foliage.
Bonemass launches a sludge ball towards you for his last attack, which his Blob minions will spawn from. You'll need to deal with these nuisances before turning your focus back to Bonemass. Don't stand near the Blobs for too long as they also release poison gas clouds of their own. If you let the Blobs accumulate, they will become a bit more than pesky distractions and pose a more serious threat, so quell their numbers as quickly as possible and get back to the task at hand.
Rewards
Once placed on its corresponding stone at the starting circle, Bonemass's trophy grants you physical damage resistance against blunt, slashing, and piercing damage.
The Wish Bone that Bonemass drops upon his death gives you the ability to sense hidden objects. Namely, this is good for detecting silver deposits up in the Mountain biomes, which delivers you to the doorstep of the fourth boss in the game.