Hammer of the Gods - Bill Willingham - Bog

Tom O'Harrow is the bastard son of Queen Mab of the Unseelie Court and True Thomas of Earth. For most of his 900 years he's been banished to our world, working as a mercenary-for-hire (provided your cause isn't completely evil). When he comes into possession of the ancient hammer Mjölnir, salvaged from the battlefields of Ragnarök, suddenly everyone is out to get him, including old friends, old enemies, both courts of Faerie, the sorcerer-scientist Daedalus, and the US Government. They want the super weapon and all agree that Tom isn't someone they can trust to decide who ends up with it.