Give the Sigil of the Hammer to Gerrig Bonegrip in the Forlorn Cavern of Ironforge.
The Tablet tells much. But not enough! Not nearly enough!
There is a book, a digest that Yagyin wrote in ages past. The ... people I fled ... they keep this book in many places. And their most learned study it. It holds vast knowledge. Knowledge to protect us!
Go to Gerrig Bonegrip. He is in a shop in Ironforge, in the Forlorn Cavern. Show him this sigil and he'll think you one of them. Ask him for Yagyin's Digest ... and bring it to me!