The Ages of the Mortal Realms or the Great Epochs refers to the primary periods into which the entire history of the Mortal Realms is organised by savants.
Before the Age of Myth
In the time before the Age of Myth, the World Before Time was destroyed and its core launched into deepest space. Sigmar clung to its Sigmarite core. This period ended when Dracothion noticed the core. He chased it, looking to capture it to admire its beauty for all time. It then noticed Sigmar, reviving him with warming breath and allowing him to ride of his back. Dracothion elevated the core to the firmament and showed Sigmar the bridges and passages that led to the Eight Realms in return for gifts of sigmarite jewellery.
Age of Myth
During the Age of Myth, Sigmar explored the Eight Realms. He united groups of human savages and fought against ancient beasts. Within generations humanity was no longer hunting with spears, but building vast cities and spires. During this time Sigmar also awoke other gods, such as Gorkamorka and Nagash. The secrets and history of this period has mainly been destroyed by the subsequent Age of Chaos, but some artifacts or architecture is still being uncovered.
Also during this time, Sigmar forged an alliance between the gods he had revived and united. This alliance was able to hold back the tides of Chaos which occasionally struck the Mortal Realms. Only when the alliance fractured did it allow the forces of Chaos to enter the Mortal Realms.
Age of Chaos
The Age of Chaos saw the destruction of the old alliance and the rise of the forces of Chaos. A series of betrayals led to the forces of Chaos being able to cross into the Mortal Realms unopposed, after which it was impossible to stop them. Eventually, Sigmar realised he could not win against Chaos alone and shut the Gates of Azyr and sealed the Celestial Realm off from the rest of the Eight Realms. He was not to be seen again for centuries. During this time, Chaos dominion was almost assured and the complete conquest of seven of the eight realms was almost complete. Only when the forces of Chaos grew tired of fighting smaller and weaker prey did they turn on themselves in titanic battles. It was at this point that Sigmar returned with the Stormcast Eternals at his side.
Age of Sigmar
The Age of Sigmar is the current age of the Eight Realms. Sigmar and his forces of Stormcast Eternals are seeking to retake the seven realms laid waste to by Chaos. He is also leading the other races in rebellion as well as seeking out older foes to settle scores with.
The first strikes were all aimed at securing the Realmgates, such as the Gates of Dawn. The Gods of Chaos bellowed with rage at this assault and all of the realms saw a surge of daemonic forces spewed forth from the Realm of Chaos. This is the current age.