Have you ever wondered whats behind that door locked by the the 3 fossilized bones? ... Well behind that door is a Black Neclace which gives you the power to hit criticals all the time.

Every now and then look at the save menu because you might be able to get your weapon stronger.

Obtain the Rekka-Ken Sword:

To obtain the ultimate weapon (the rekka-ken),you'll need to get near to the area where Nobunaga is after you get the purple wedge. Enter the Phantom Realm after using the purple wedge on the area and go all the way down to the bottom floor.There are about 20 floors to get through but it is easy.After you beat the last batch of monsters on the ground floor.The Rekka-Ken would automatically appear and you will obtain the sword.

Also try:

after getting the sword push x on the area where the sword was. you'll have the option to plunge the sword back in. Do so then get sucked into the realm and there would be a box. Open it and you would get Jubei's second costume and you'll still have the sword.