Ghal-Maraz
Ghal-Maraz is Sigmar's main weapon and, according to some, the "Warhammer" of the franchise, as such it bears special symbolism for all things Warhammer.
Warhammer Fantasy
It was originally forged by the dwarfs and given as a gift to Sigmar for saving the then High-King of the Dwarfs, Kurgan Ironbeard from a mob of Orks, it became the symbol of the Empire and the fav weapon of the Emperors. During The End Times Sigmar managed to be reunited with his hammer after this was briefly captured by the forces of Chaos, allowing him to outmatch Archaon in battle, sadly this wasn't enough to save Mallus (the Warhammer world) with both Sigmar, Ghal-Maraz and Archaon falling through a warp-rift which consumed 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.
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 succeded and Sigmar gave the hammer to the Celestial-Prime (we still don't know his true identity but everyone wishes it was Settra the Imperishable or Karl Franz). The Celestial-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 don't try this with Archaon), needless to said, Nurgle needed a lot of ice after that incident.