Ghal Maraz ("skull splitter"), the Hammer of Sigmar, was wielded by the famed founder of the Empire, Sigmar Heldenhammer. It was gifted to him by the Dwarf King Kurgan Ironbeard after Sigmar rescued him from a band of Greenskins. According to Alaric the Mad the hammer itself was crafted by ancient Dwarfs, and the skill, power and patience put into its creation was never possible again, effectively making Ghal Maraz unique in its power. Similar to many other Dwarf weapons, it contains immense power in the form of Runes.
When Sigmar abdicated he left Ghal Maraz behind as a symbol of the Emperor's authority. It has since been passed down from Emperor to Emperor, and is currently held by Karl Franz. During the Storm of Chaos however, Karl Franz granted the hammer to Valten, Sigmar's champion, to wield. Upon Valten's death the hammer presumably passed back to Karl Franz.