Bonemaw

Bonemass is a swamp boss in Valheim and one of the game's Forsaken. Its Forsaken Altar is found in the Swamp, and its location can be revealed by a Vegvisir discovered in Sunken Crypts and sometimes in ruined structures. It is summoned by placing 10 Withered Bones on its altar, and like all bosses it can be fought again as many times as the required materials are available.

Bonemass is a slow but extremely dangerous close-range enemy that relies on poison, heavy blunt attacks, and summoning support. Its presence also changes the local weather, creating rain in the area and making it difficult to remove Wet by dragging the fight elsewhere. The boss can attack in several distinct ways:

• A close-range swipe that deals up to 80 Blunt and 50 Poison damage, and can also damage trees and rocks.
• A bite attack for 140 Slash damage when the player is very close.
• A poison vomit attack that creates a persistent cloud in a 9-meter radius, dealing 130 Poison damage and continuously refreshing Poison on anyone who remains inside.
• A goop throw that can summon four random monsters, chosen from Skeletons and Blobs.
• A fireball-like spit attack that launches three projectiles causing Burning and Poison, though the sources describe this as a separate ranged move used at medium distance.

Its poison cloud cannot be dodged or blocked, so the safest approach is to avoid standing in the cloud at all. The cloud does not extend very far behind Bonemass because of the boss's collision box, which creates a relatively safe retreat point directly behind it while still leaving room to continue attacking. Bonemass also tends to keep moving toward the player even while in range, so maintaining spacing is important. Because its summons can continue even when spawn caps are reached, the fight can become crowded if the battle drags on.

Defeating Bonemass rewards the player with the Wishbone, which is used to locate buried Silver veins in the Mountains, buried Muddy Scrap Piles in Swamps, and other hidden treasures. Its trophy can be mounted on the Forsaken Stone to grant the Bonemass Forsaken Power for 300 seconds. This power provides the player and nearby allies with slightly reduced damage from Blunt, Slash, and Pierce attacks, removes block stamina usage, and increases block stamina return.

Defeating Bonemass also affects world events. It disables the draugr and skeleton version of “A foul smell from the swamp...,” enables the blob and oozer version of that event, and unlocks several other events including “A skeleton surprise!”, “You are being hunted.”, and, depending on world progression, other biome and creature events. It also unlocks Skeletons as nighttime spawns in several biomes, including the Meadows, Black Forest, Swamp, Mountains, and Plains, though they remain unstarred.

A few practical notes are worth keeping in mind. One source notes that Bonemass has an environment aura that forces rain, which makes the fight consistently wet and reinforces the Swamp's intended challenge. Another notes that the in-game trophy menu includes an alternate description of the boss. There is also an unused asset named BonemassSerpentScale that can only be spawned with the developer console and has no known purpose.