The Sundering refers to the event far beyond ancient history, when the 12 segments of the Ring were forcibly separated from each other. It is assumed that Urohringr was not originally meant to be separated in such a way; whether it was done maliciously or out of some attempt to save the world is yet to be revealed. From what is known about the Herald, it is assumed that the Sundering itself was orchestrated by ancient pegasi operating aboard Sentinels.
The event itself is described multiple times in the series:
- During Rainbow's conversation with Nevlamas: "But as she came closer, there were multiple explosions. With a burst of lavender light, what was once one became twelve, and they spread apart like leaves across a frozen black pond." (Odrsjot ch. 83). Nevlamas is convinced that "Whatever caused the Sundering, it was simply hungry... craving pain and agony and suffering."
- In Rainbow's vision near the beginning of Yaerfaerda: "One ring drifted into an incalculably large cloud of chaotic space. The light that fueled the heart of the structure dissipated. Then, after a flash of discordant energy, the ring split into twelve different segments. They all flew in opposite directions, but one plane in particular flew even deeper into the miasma." (Yaerfaerda ch. 2) Note that in this instance, the mysterious voice mentions that "...there are also those who do the wrong things for all the right reasons", which may be referring to those who orchestrated the sundering.
While Pilate and Rainbow are exploring the sacred chambers beneath Durandana, they hypothesize that the enormous crater around the city - as well as other geological phenomena - may be attributed to the upheaval that accompanied the Sundering itself.
In the machine world at Seclorum's Camp, Rainbow Dash experiences a vision in which she sees ancient pegasi placing the flames on the pedestals, which was apparently the actual means which accomplished the Sundering.
Madame Nightshade, while referring to the Ruby Flame, states that "...[the flame's] only purpose is sundering". (citation needed)