Overview
Bonemass is a Valheim world location that spawns in the swamp biome, identified internally as Bonemass. World generation places up to 5 per world, with terrain constraints (altitude 0-2, max delta 4), and an exclusion radius of 19.79m, which is why some seeds have far more of them than others. The data tags it as prioritized.
About
Bonemass is Bonemass's Forsaken Altar in the Swamp. Ten Withered bones looted from Sunken Crypts are placed on the central stone to summon Valheim's third boss.