While in the Lhusu Mines, far into the game, or at the Gilgamesh Mark Hunt, go to the bridge that is pointing South-West (or South-East). It's a diagonal. Anyway, enter and re-enter in a few times until you see a black Bomb (Bullitt) appear. Kill it, and you'll have a chance of getting the Yagyu Darkblade (called the Yagyuu's Onyx in the Japanese version).

You cannot go here until the get the Area 11 key, which is conveniently located in the Phon Coast and the Hunter's Camp, somewhere around one of inhabitants (look for the sparkiling object on the ground). Note: you must have done the Antlion mob first.