User blog comment:JAlbor/Dawnguard: What Everyone Should Know/@comment-1080767-20120628025347/@comment-1080767-20120629015526

The hammer is called a Dawnguard Rune Hammer, if I recall, and Florentius gives you a quest to retrieve it, as well as a shield later that is like Spellbreaker, but instead of having a magic protection effect, it has a magic damage effect.