In the Warhammer Fantasy setting, Sigmar is the primary god of The Empire. His sign is a two tailed comet. About 2500 years before the present time, he was a mortal who united the tribes of what is now the Empire.
The Imperial Calendar (IC) which takes its starting date from Sigmar's coronation as Emperor, places his birth as the year -30, in the Reikland area in the southwest of the Empire, to the Unberogen tribe, generally considered to have been one of the most powerful pre-Empire tribes. The night of his birth was marked with the appearance of a twin-tailed comet, which the human tribes took as a sign of great portent from the gods. In the year -15, Sigmar is believed to have helped drive off a Goblin invasion of his village, and in the same year, led a punitive expedition against a Goblin war-party that was holding Kurgan Ironbeard, a king of the Dwarf people, a prisoner. In his gratitude, Kurgan granted the young warrior with the magic rune-enchanted warhammer, Ghal Maraz ('Skull-splitter,' in the Dwarf Khazalid tongue).
Sigmar then went on a campaign to unite the disparate tribes of the future Empire, and one by one they submitted, either by conquest or diplomacy. The most famous incident was his subjugation of the belligerent Teutogens, the largest and most powerful of the tribes, who lived near the Middle Mountains in the north central Empire, near the current location of the Middenheim City-State (Freistadt). Their chieftain, Artur, was defeated by Sigmar in single combat, and this is commonly held to have been the point at which Sigmar gained control of the tribes of the Empire.
From that point onward, Sigmar embarked upon a campaign of purgation and liberation throughout all of the tribal lands, primarily against the Beastmen and Goblinoid races (Orcs and Goblins), culminating in the First Battle of Black Fire Pass, in present-day Stirland, in the southeast of the Empire. Following this great victory, Sigmar returned in triumph to his native Reikland and was crowned Emperor Sigmar Heldenhammer I('Hammer of the Goblins') at Reikdorf, the site of the current Imperial capital of Altdorf. This date remains the Empire's greatest holy day, as it marks Sigmar's coronation and abdication fifty years later (IC 50) - it is in the summer of the Imperial year, on the 18th day of the month of Sigmarzeit.
Sigmar set up the leaders of the twelve tribes that followed him as the Elector Counts of the Empire, a position that remains to this day. The Runefang swords, commissioned by Sigmar to the legendary Dwarf smith Alaric the Mad, were not finished before Sigmar's abdication, but nevertheless remain among the most potent symbols of Imperial rule.
In his fiftieth year of reigning, Sigmar put by his crown and set off to see what lay beyond the World's Edge Mountains to the east, in a similar fashion to the Dwarf ancestor/warrior deity Grimnir the Fearless, who legendarily strode off into the Chaos Waste to do battle with the Powers of the Warp. Sources conflict on the details of what precisely motivated Sigmar to do this; some accounts claim he set off to return Ghal Maraz to the Dwarfs, but the famous rune-hammer has traditionally been the weapon of the ruling Emperor and the primary symbol/relic of the Sigmarite cult, founded by Johann Helstrum in IC 73. A number of heresies have sprung up regarding the authenticity of Ghal Maraz, but the official Imperial position is that the weapon wielded by the Emperors is the hammer of Sigmar.
Following his disappearance, Sigmar passed over the World's Edge mountain range and no human ever saw him again. Since he was never known to have died, this may have played a large role in the early successes of the cult established in his honor. It is now the foremost religion in the Empire and is inextricably intertwined with the political, cultural, and national identity of the Empire and its people. Interestingly, Sigmar never claimed to be a deity and his own religious beliefs while alive are subject to conjecture, although tradition dictates that he was crowned Emperor by the High Priest of the cult of Ulric, the northern god of winter, wolves, and war. However, the priests and worshippers of Sigmar often receive measurable and often positive answers to their entreaties and petitions, suggesting that something with some measure of power is answering them. Many take this as a sign that Sigmar indeed is a god, and watches over the Empire.
Surprisingly (or perhaps not at all), the cults of Sigmar and Ulric do not get along well. The Ar-Ulric and the two Arch-Lectors of the Sigmarite faith, as well as the Grand Theogonist (current: Johann Esmer), all maintain a vote in the election of the Emperors. While the cult and its leaders are often corrupt and hip-deep in the byzantine politics of the Empire, it is a faith that preaches courage, justice, honor, and the protection of the weak and innocent from evil. It is also nationalistic and sees the preservation of Sigmar's original holdings as a sacred duty. The church's iconography primarily centers around images directly connected to Sigmar himself: the comet and the hammer are the foremost of these. Also a popular national and religious symbol is the griffon, the personal heraldry of Emperor Magnus the Pious, who saved the Empire from disaster during the Great War Against Chaos in IC 2302-2303. It is likely that Magnus will be canonized as a saint within the next century (current year: IC 2522).
A popular fan theory is that Sigmar is one of the two missing Primarchs in Warhammer 40,000.