History: Beacon World Era
The Beacon World Era is the name of an almost half a million year long period of time in the history of the galaxy, which started 30 thousand years before the first Beacon World Activation Event, when the Aurost and the Sun Serpents arrived to the galaxy the first time (and also when the last of the Shapers ascended) and ended with the Serpent Resurrection.
The age of the Shapers was coming to an end, and the Cloudan was well aware of it. They were impossible to encounter at any point, though, so the Cloudan focused all its resources to find the single entry point to the Gate, the final major Shaper construction they found to be still active.
Regh: the Aurost homeworld
Due to the inaccessibility of outer locations, it is still unknown, whether the destination point of the Gate was actually Regh itself. Most of the sources from the age claim that what happened at the point of the Activation Event was practically a Cloudan-Aurost exchange: while the invaders attacked the other side, some could find a chance to slip through, and thus, being the first of the Aurost of the galaxy.
Since it is unknown how the Gate works, historians have no idea, not even a guess, whether this theory is plausible at all. According to the current level of understanding on Shaper technology, opening a spatial warp on such distances that would actually provide access for thousands, if not millions of lightyears is unfeasible, and as such, impossible.
What happened and what was on the other side of the Gate in BY1 is one of the biggest ever unsolved mysteries of the galaxy.