The Klein Bottle.

The Klein Bottle that you can find in certain variants of CLIVAN is generally used in a interesting quest line that seems to be incomplete.

If you go into The Forest section; and go far enough, I believe you can fight a Vampire... Which, you can find a special staircase. This leads you to a small chamber, with what seems to be a Priestess of Scabies, who has a special quest for you. You are given a Klein Bottle, that is filled with blood... You must go, locate the Enner Beast, and get close enough to him that his shrieks cause the blood to coagulate. You go back, and sadly... I can't recall what happens. I think in my current game, I had killed the Enner before hand.

Oh! Now that you mention it I do vaguely remember that quest though similarly I can't remember what it did.
Time to go check so I can update the wiki I guess.

EDIT: Ok, that was easy enough. Completing the quest for the witch (priestess of Scabies) grants you the map to the Tomb of Xinroch, which is currently unfinished. Time to go update the wiki, thanks Zayre for bringing this up.