Bane burial longsword is a piece of burial equipment that can be made at a burial forge and anvil. Making this item consumes a bane longsword + 4 to make the unfinished smithing item. Once the progress on the object has been completed, the item is immediately removed from your inventory. This effectively functions as 'no reward high experience' method.
Burial items grant the same amount of experience as the final upgrade of that item. This effectively doubles the experience per item if a burial item is made from a +3 item, or a 30% increase if the burial item is made from bars.
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