Ghal-Maraz
Ghal-Maraz ("skull-splitter" in Dwarfish) was the main weapon of Sigmar, and according to tradition, was passed down to all the Emperors of the Empire of Man. It also serves as the namesake, and part of the logo, of the Warhammer franchise. As such, it bears special symbolism for all things Warhammer.
Warhammer Fantasy
It was originally a two-handed hammer forged by the Dwarfs for the Dwarf king Kurgan Ironbeard, but the dorf gave it to Sigmar as a gift for saving his life and those of his kindwarfs from a mob of Orc Marauders. It later became the symbol of the Empire, and the favored weapon of its various Emperors. It has some kind of magical power due to the Dwarven runes engraved on its haft and head, which helped out considerably on many occasions; such as when Sigmar fought the leader of Middenland and the hammer was blessed by Ulric after said leader threw Sigmar into Ulric's flame, only for him to come back out completely unharmed, roaring like the fucking savage he was before shattering the leader's precious long sword (aka the Dragon Sword of Caledfwlch, a weapon made out of lightning frozen by a frost drake and was given by a stranger from across the sea), along with his precious skull. Another occasion was during the last battle against Nagash,the Arch-Necromncer of the Warhammer world. Sigmar and the big not-quite-as-likeable-desert-daddy fought for hours, until Sigmar raised the weapon to strike, and Ghal-Maraz then began to glow faintly as the runes engraved into it reacted to Nagash’s foul sorcery. The resulting magical energy gave Sigmar’s blow power enough to knock Nagash right the FUCK OUT MAN!
After Sigmar left the empire on his last adventure, the hammer was passed down from Emperor to Emperor all the way to Karl Franz who then used it to smash many Orcs at Blackfire Pass. The weapon was then passed to Valten after many people began to believe that he was the reincarnation of Sigmar himself. The abilities of the hammer itself were a closely guarded secret amongst the clerics of the Church of Sigmar, since the Hammer that Franz and Valten wielded proved to be much less potent than what was told in myths and legends (albeit still far above even some of the most powerful magical weapons known to Imperial scholars at the time), and there was hot debate over if the Ghal-Maraz still possessed by the Emperors was the original; some claiming it was a well-made forgery and the original remained with Sigmar whereever he might have gone, and others claiming historical exaggeration explained the disparity in power between myths and reality.
During The End Times, Valten carried the hammer to smash many a Chaos prick (Notably: Crom the Conqueror, Wulfrik the Wanderer (peace the fuck out at Slannesh's realm, you red haired bastard!), Count Mordrek the Damned and a changeling that impersonated Gelt's disciple) while on his way to Middenland. He was ultimately killed by a backstabby Vermin Lord Deceiver after being exhausted from his duel with Archaon. Ghal Maraz was in Archaon's care for a while, until Sigmar managed to reunite with his beloved hammer. He then smashed Ka'bandha's face in with it. It allows him to outmatch Archaon in battle, but it was not enough to save Mallus (the Warhammer world), with both Sigmar, Ghal-Maraz and Archaon falling through a Warp-rift which then proceeded to consume the planet.
Both Sigmar and Ghal-Maraz managed to survive the apocalypse and were merged with the essence of the Realm of Heavens, becoming even more powerful than ever. Unfortunately, Tzeentch tricked Sigmar into throwing Ghal-Maraz into another warp-rift while trying to strike down Archaon, which ensured Sigmar's withdrawal and the coming of the Age of Chaos.
Note that the hammer's canonical appearance has the traditional medieval war hammer look, with its head composed of a heavy iron hammer head in front, and a curved spike on the other end. It DOES NOT look like a maul, which has heavy iron hammer heads on both ends. Some novel covers like to portray it as a maul, like that one cover where Sigmar fought against Nagash, which is flat out wrong. Despite it being made for a Dwarf, it's long enough to be wielded using both hands, even by Men.
Age of Sigmar
With the coming of the Age of Sigmar, one of the first tasks the God-King gave to the Stormcast Eternals was to recover Ghal-Maraz. They succeeded, and Sigmar gave the hammer to the Celestant-Prime (we still don't know his true identity but people think it is Settra the Imperishable, or Karl Franz). The Celestant-Prime, now named after the hammer, managed to rescue Alarielle the Everqueen of the Realm of Life from being caught by Nurgle, while in the process defeating Chaos Lord Torglug the Despised, killing the chaos guy like the failure he is No wait! You thought it was slay-the-heretic-inquisition-time as usual? Ghal-Maraz is presently imbued with the power of Noblebright, and it purged Torglug, bringing him back as a Stormcast Eternal. That's right kids, Sigmar's main weapon can pull back mortals from corruption (as long as there is still some sliver of soul to be saved, so you better not try this with Archaon. Torglug, now Tornus, was exceptionally resistant, holding out against Nurgle's tortures for seventy seven days before snapping). Needless to say, Nurgle needed a lot of ice after that incident.
Tabletop
Ghal-Maraz was wielded by only two people on the tabletop prior to Age of Sigmar: Karl Franz, and Valten. Both got the same use out of it: all hits automatically wound, allow no armor saves and deal d3 wounds. This means that you roll to hit, then roll 1d3 per hit to see how many wounds your opponent eats. Given that both Karl and Valten had pretty high WS and Attack stats, this means that in a duel they wreck face like it's nobody's business, and could deal massive damage against infantry hordes, especially when paired with either a monstrous mount (like Karl Franz), a large support unit (Valten) or even both if you can afford it.
