The dwarfs are known for having the most well known book in the Warhammer world; The Book of Grudges. There is one that the High King holds that encompasses the entire Dwarf race, but each dwarf city has their own books. Thus, I think it would be fun and fluffy if it was an upgrade option. Here is my idea:
The Book of Grudges
Of all races Dwarfs hold grudges the best, and take them so seriously that they have a book to keep track of these wrongdoings against them. In times of battle the bearer of these books has been known to take to the field. Held in his hands is one of the books, during the fight he bellows out the wrongs caused by the dwarfs opponents against them, whipping the Dwarfs into a rumbling hatred.
The Book of Grudges is an upgrade option for one Thane in a Dwarf army, at the cost of +50pts.
The unit in which the bearer of the book leads has the Eternal Hatred rule (hated lasts for all turns of combat), and in addition any dwarf unit within 6" gains hatred against all of the opposing army (normal hatred).
What do you think? I was considering allowing him shieldbearers which would increase the range of the Hatred, maybe to 12"? I don't think he's all that powerful really, and I don't know if he's too costly - 25pts or 50pts?