Bonemass is a boss with 5,000 health.
It takes extra damage from blunt and frost; it resists slash, pierce and fire; poison does nothing to it at all.
Its hardest hit is aoe at 130 poison, out of 2 attacks.
Some of what it throws is elemental, so poison resistance is worth having.
You cannot block aoe, so give yourself room to dodge instead.
Of everything craftable, 
Frostfire Sledge, 
Thunderblood Sledge and 
Frostfire Mace put the most damage through what it resists.
It always drops 
Bonemass Trophy and 
Wishbone.
Bonemass sleeps under a skull-marked stone in the Swamp. Vegvisirs in Sunken Crypts reveal the altar. Offer ten 
Withered Bones, dug from muddy scrap piles and looted in the crypts themselves.
Bonemass is a gear check for poison. His vomit cloud deals 130 poison damage in a 9 meter radius, cannot be blocked or dodged, and lingers for 15 seconds. Without 
Poison Resistance Mead you will die. With it, the fight becomes a patient grind: he is weak to blunt and frost, shrugs off arrows and fire, and an 
iron mace to the face is the whole strategy. When he reaches into his side he is about to throw a glob that hatches skeletons and blobs; cut the adds down fast or drag him away from them. His aura brings constant rain, so stay off the water and count on being wet.
The tank power. Most players keep it slotted for every hard fight that follows.
He drops the Wishbone. Wear it and it pings buried silver veins in the Mountains, muddy scrap piles in the Swamp and hidden treasure elsewhere.
With devcommands enabled, this spawns Bonemass in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: Bonemass · all console commands and IDs