If you want to progress in Valheim, you need to muster up enough resources to battle against its bosses. Each of the bosses drop a particular that you need to advance in the game. For those looking to locate silver, you need to acquire the wishbone, and it only drops off of the third Forsaken boss, Bonemass. The only way you can summon Bonemass is by dropping off 10 withered bones at its altar.
Withered bones are hidden within the Swamp crypts. These crypts are hidden throughout the Swamp biome, and the only way to access them is to have a Swamp key. The Swamp key only drops off of the Elder, Valheim’s second boss, which means you need to defeat them before attempting to take on Bonemass. While the Elder only drops a few keys after you defeat it, the Swamp crypts do not cause the keys to disappear from your inventory. You can use a single key to open all of the crypts in your Valheim game, so there’s no need to farm the Elder.
When you enter the Swamp crypt, there are withered bones in multiple locations inside. You can find them in chests, on the ground, or by mining the large muddy scrap piles. These scrap piles are blocking most of the passages within a Swamp crypt, and the only way to bypass them is with a pickaxe. You can use any of the pickaxes, but we recommend using a bronze one, or better. We also recommend you carry multiple with you to avoid making frequent trips back to your forge to repair the one you’re using.
From what we can tell, the withered bone has no other use than summoning Bonemass. You can keep as many as you want at your homestead, making it easy to summon and battle Bonemass whenever you wish.