That person is Brynden Rivers, aka Bloodraven ... then its very likely the sword remains in the cave with him, Bran, Meera, Hodor, and Summer. If Brynden did indeed take Dark Sister with him ...
One of the characters we see in the visions is Brynden Rivers, and though he wasn’t named, you could clearly see he was a Targaryen and was curiously embedded into a tree just like the Three ...