Valheim: How to Summon the Bonemass Swamp Boss

Bonemass is the third boss in Valheim and the guardian of the Swamp biome. He never spawns on his own. The fight only begins when you walk up to his Forsaken Altar, a giant stone skull venting green gas, and feed it the one item it wants.

Quick answer: Put 10 Withered Bones in your hotbar, aim at the Boiling Altar inside the stone skull’s mouth in the Swamp, and press that hotbar number to make the sacrifice.

Bonemass summon requirements in Valheim

Everything in the chain depends on one earlier boss. The Elder, who lives in the Black Forest, drops the Swamp Key, and without that key the crypts holding Withered Bones stay sealed.

Requirement	Amount	Where it comes from
Withered Bone	10	Sunk Crypts in the Swamp
Swamp Key	1	Dropped by The Elder
Forsaken Altar	1 location	Swamp biome, revealed by Vegvisir runestones

Withered Bone is a heavy material at 10.0 weight per unit, described in the inventory as “A giant bone, knotted like old wood.” A full set of 10 is 100 weight, so clear inventory space before you start mining.

Find the Bonemass altar location in the Swamp

The altar is a huge skull rising out of the ground, lit by a green glow, and it only appears in Swamp biomes. You do not have to stumble onto it by luck. Vegvisir runestones inside Sunk Crypts, and occasionally in ruined structures out in the open, mark the nearest Bonemass summoning site on your map.

Because the Swamp forces the Wet status on you constantly, expect a slower stamina regeneration and a colder character the whole time you are exploring. Pin the altar on your map the moment a runestone reveals it so you can walk back with a full load of bones later.

Player wading through the rainy Swamp biome with the Wet status icon showing in the HUD — Travelling through the Swamp with the Wet status active while searching for the altar. Image: Iron Gate AB

Open a Sunk Crypt with the Swamp Key

Defeat The Elder at his altar in the Black Forest. He drops the Swamp Key, which is the only thing that opens crypt doors.

Search the Swamp for a Sunk Crypt. These are squat stone buildings half-buried in the muck, sealed behind a barred Iron Gate.

Stand at the gate and press the interact prompt, shown on screen as [E] Open. As long as the key is on you, the game confirms with “Using key: Swamp Key” and the gate swings open.

Player standing at the barred Iron Gate of a glowing Sunk Crypt entrance in the Swamp — The barred Iron Gate of a Sunk Crypt, which only opens with the Swamp Key. Image: Iron Gate AB

The key is not consumed. One Swamp Key opens every Sunk Crypt you find, so you never need to farm The Elder again for more.

Farm 10 Withered Bones inside the crypt

Once you are inside, the bones come from two sources, and both need to be broken apart rather than picked up.

Muddy scrap piles — mine them with a pickaxe; they yield Withered Bone alongside scrap iron.
Body piles — destroy them with a weapon to shake out bones.
Drops arrive in small stacks, with the pickup message reading “Added Withered Bone x2”.

Bones do not all come from a single crypt reliably. If one dungeon runs dry before you hit 10, note where the next crypt is on your map and keep going. Later on, the Wishbone that Bonemass himself drops will detect buried Muddy scrap piles out in the Swamp, but that only helps on repeat runs.

Player moving back across the Swamp with the first hotbar slot selected and a Wet status effect — Heading back to the altar with the bones loaded into the first hotbar slot. Image: Iron Gate AB

Offer the bones at the Boiling Altar

Clear the ground around the skull first. Draugr, skeletons, blobs and leeches wander the Swamp freely, and Bonemass summons more of them once the fight starts.

Move all 10 Withered Bones into one of your hotbar slots. The offering interface reads from the hotbar, not from your main inventory grid.

Walk into the skull’s mouth and look at the fire pit inside it. The interaction prompt names it the Boiling Altar and shows [1-8] Offer item.

Press the number key matching the slot holding your bones. All 10 are consumed in one action.

How to confirm the summon worked

Two messages appear in quick succession. First “Removed Withered Bone x10” logs the payment, then the screen reads “You make the sacrifice”. Bonemass rises out of the ground shortly afterwards and his health bar appears at the top of the screen.

If the prompt never shows [1-8] Offer item, you are either standing outside the altar’s interaction range or the bones are sitting in your inventory rather than a hotbar slot. Nothing is spent until the sacrifice message appears, so a failed attempt costs you nothing.

The altar is reusable. Bonemass can be summoned as many times as you like, and each additional summon costs another 10 Withered Bones, which means another trip into the crypts. Bring a portal to the Swamp before you commit to the fight so the walk back is not part of the cost.