This is part of an extra quest called... "The Mysterious Quest!" There actually 4 items you will need. The bucket, the medallion, the book, and the stone. The first 2 you have. The book you could likely get quickly and the stone is a BEE ATCH. The book is acquired by returning all 20 tomes to the guy in Kalrathia that asks you to collect them for him.(can't remember his name?) There are walkthroughs out there to tell you the exact spot they books are if you don't have them all. The stone can be somewhat more troublesome. Ever see that little ??? bugger? well you need to kill him in 3 shots or less to get the stone. After that he disappears again. The trick is to get him to come out and play!! There really is no easy way to do this. The best bet is to beef up on % magic find items and find a densely populated monster mob and start wacking. Don't pick up the items that fall because these are what attracts ???, notably very rare's and above. The only time I did this quest it took me many hours to lure him out and successfully eliminate the little bugger. Some people have better luck, but its hit or miss. Others things to watch out for is your buffs. If you use an offensive one, turn it off. It will act as one hit against ??? and make it that much harder for you to kill. It pretty much takes 3 power attacks to kill it.
Once you achieve all the above then you are able to enter the mysterious cave and complete that quest. To enter the cave you need to speak a special chant at any shrine to teleport you there. Unfortunately I can't remember it off top of my head, so again either find a walkthrough or someone else will fill it in for you. Hope I was of some help anyways.
I crave this game like a fatboy to a smartie.