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 kindwarf's from a mob of Orc Marauders. It later became the symbol of the Empire,and the favorite weapon of its various Emperors. It has some kind of magical power due to the Dwarven runes engraved on it’s 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 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 fight for hours in end before Sigmar raises the weapon to strike.Ghal-maraz then begins to glow faintly as the runes engraved into it react to Nagash’s foul sorcery.The resultant instability 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 black fire pass.The weapon was then passed to Valten after many people began to believe that he was the reincarnation of Sigmar himself.
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 hair 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. The hammer 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.
Also, noted that the hammer's canonical appearance has the traditional medieval war hammer look, with it's head compose of heavy iron hammer head in front, and a curved spike on the other end, and it DOES NOT look like a maul, which has heavy iron hammer head on both end. Some novel cover likes to portray it as a maul like that one cover where Sigmar fought against Nagash, and that is just wrong. Despite it is made for a Dwarf, it's long enough to be wield using both hand, for Humans even.
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), needless to say, Nurgle needed a lot of ice after that incident.
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 Franz 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.