The Warhammer universe possesses several different alternate timelines. This article tries to help readers navigate between them.
Storm of Chaos
The Storm of Chaos (2521-2522 IC) was the name later given to the greatest assault launched by the servants of Chaos upon the Old World, when Archaon the Everchosen and the forces of Chaos sought to conquer first Kislev and the Empire, and then the rest of the continent. Archaon was thwarted in this attempt by the valiant efforts of the Men of the Empire, and the End Times were averted, at least temporarily, in this timeline.
Age of Reckoning
The Age of Reckoning timeline takes place in the aftermath of a great plague when the Empire, the Dwarfs, and the High Elves joined forces to battle Chaos, the Dark Elves and the Greenskins. In this timeline neither the Storm of Chaos nor the End Times ever happened.
The End Times
The End Times (2519 - 2528 IC) is the great cataclysmic apocalypse that was predicted to bring about the end of the mortal world. It was heralded by the arrival of the Twin-Tailed Comet in 2519 IC, both an omen of hope and one of destruction.
The comet signaled the coronation of the thirteenth Everchosen of Chaos Undivided: Archaon the "Three Eyed King," the "Lord of the End Times," and his bid to bring destruction upon all the realms of Men in the Old World. As the world endured its final, agonising demise, the mortal races of the Old World fought valiantly to the very last against the encroaching darkness which sought to consume them all.
Unfortunately, despite their great efforts, the peoples of the mortal world failed to stem the tide of Chaos and the Known World was destroyed as the full power of Chaos was finally unleashed and unmade all creation.
Further non-canon information providing alternative endings to the End Times can be found here: Alternative End Times Timelines
Gotrek and Felix Storyline
Gotrek Gurnisson, a Dwarf Slayer, and Felix Jaeger, his Human chronicler, are a duo of warriors travelling throughout the length and breadth of the Old World, battling dark forces and stopping evil plots in Gotrek's quest for a heroic death against a terrible foe. The adventures of these warriors have been written down in the series of books known as My Travels with Gotrek by Felix Jaeger, which outlines the unlikely pair's many adventures, recording everything that transpired between them and to them until the day Gotrek finally met his doom, and was finally allowed to enter the halls of his ancestors.
In the last few novels about Gotrek and Felix, starting from Orcslayer all the way up to Zombieslayer, the events take place during and after the Storm of Chaos. However, by the time of the novel Kinslayer, which takes place after Zombieslayer, the timeline has shifted and now the two adventurers are present during the early years of the End Times. This was a deliberate retcon as Games Workshop changed the Storm of Chaos into a non-canon timeline in favour of the now-canonical End Times.
Guardians of the Forest
Leofric Carrard came out of Athel Loren 100 years after he first entered due to the time dilation effects that plagued many of those who entered that magical forest. He emerges to meet his own great grandson who is himself an old man and the lord of Castle Carrard in around the year 2622 IC, long after the world was already consumed by the End Times in the primary timeline. This means that this alternate timeline takes place farther in the future than any of the others in the Warhammer universe.
Total War: Warhammer
The three computer games of the Total War: Warhammer franchise take place in a somewhat different variant of the Warhammer Fantasy Battles universe, and includes several canonical changes made for gameplay reasons such as the continued existence of characters like Repanse de Lyonesse into the early 26th century IC, and a different Elector Count lineup for the Empire during the election of Karl Franz as emperor that is identical to that following the Turmoil of 2512 rather than the one canonically present before that short Imperial civil war.
The games also had narrative campaigns in Total War: Warhammer II and Total War: Warhammer III which included the possibility of outcomes never explored in the original canon timeline, such as the Horned Rat, the minor Skaven Chaos God, entering the mortal world through the Great Vortex and the Kislevite bear god Ursun being chained by the Daemon Prince Be'lakor in the Forge of Souls.
Age of Sigmar
Warhammer Age of Sigmar is the spiritual and in-universe successor to the Warhammer Fantasy Battle and Warhammer Fantasy Roleplay universes, after the setting was discontinued by Games Workshop in 2015.
Due to this change, Warhammer Age of Sigmar contains many of the same characters, themes, and models as its predecessor and canonically directly follows on from the events and timeline of the End Times, though many aeons later. In the Mortal Realms of the Age of Sigmar the Warhammer World is remembered only as the "World-That-Was," though many of its greatest heroes have been reincarnated in new forms.