Bitterblack weapons are found in cursed form and are unusable until they have been purified. The higher the level, the greter the cost to purify them - a Level 1 weapon costs 5,200 RC to purify, whilst a Level 3 weapon costs 28,800 RC to purify.
Level 1 weapons are obtainable in Gransys, though often rare. Level 2 and 3 weapons are not available outside Bitterblack Isle.
Only unpurified weapons can be traded, the purification results cannot.