The Darkstone Tablet holds the writings of the long-dead dwarf Urheld Darkstone. An alchemist of great skill, he was thought insane by his contemporaries and, hence, did not share his research.
But he did etch it onto tablets, and I believe one such tablet lies in Blackrock Spire, in a chamber once named the Hall of Fortune. I know not what it is now called, for few enter Blackrock and return.
But if you are willing to brave its dangers and return to me with the tablet, your reward will be great.