Saemon stole the silver blade, and the people he stole it from want it back. A war party will eventually track you down (probably when you least expect it). For some strange reason, they have the silver hilt. If you take both pieces to Cromwell, he can assemble the Silver Sword for you.
BTW, the leader of that war party is a character created by a fan who won the chance to have his character appear in BG2. He's pretty funny.
