Eidolon

While most beholders have little interest in the history of their race, there are enough historians among them to have carried forth legends about Eidolon. This planet was believed to be the origin world of the beholder race.
At some point in the distant past, the world was destroyed with little prior notice. This occurred at a time after the beholders had been a space-fairing race for some time. In theory many were present off-world, and those on Eidolon could have had access to spelljammers with which to escape.
There is little consensus as to the source of the destruction. Before the modern era, the beholders were a scattered race, with individuals and small groups living in the fringes as little more than savages. Thus, they did not retain records for themselves. A destroyed homeworld and resulting decline would explain the state they were in before the formation of the Ocularum.
Some ancient records found in collector's libraries pair the beholder diaspora and the first sightings of the Astral Dreadnaught. This could be the reason that the Ocularum offers such a high bounty on information related to the Dreadnaught's movements.