Is there any way to break your oath to return the blade? Any negative consequences? I tried just giving the sword to the ghost ship lady (nothing to say to the death guard in the crypt), exchanging the sword for the ship (the death guard in the crypt gets pissed and attacks), and going back with the sword after getting the ghost ship and I can choose to return the blade (or decline for now, which he suggests bad stuff will happen if I don't give it back soon). But I then went to Ukaizo with no apparent ill effects.
So rather curious about that.