All started circa 1384 DR when an eye tyrant clan ship crash-landed into a deep ravine in the Dragonspine Mountains. Of the thirty-five beholder passengers, only fifteen survived. They used their eye rays to tunnel deeper in order to escape from the crashed ship and at last emerged in Mulmaster's sewers.
They remained there, plotting their next move, and dreaming of maybe conquering the city. However, an adventuring party discovered their lair and caused the cavern to collapse on both adventurers and beholders. Only three beholders, Xorack, Xamott, and Xanshin survived.
The three waited for a century, trying to come up with plan after plan to seize power while slowly tunneling their way out of their underground prison with their disintegration eyes. Finally, they emerged with a plan: summon a dozen spectators in order to impersonate the other beholders. So the newly assembled Mulmaster Beholder Corps became a notable organization in the city, with everyone opposing them believing they had to face fifteen beholders together. However, people would only ever deal with three "representatives" of the group.
Xorack, Xamott, and Xanshin's main goal was to become a powerful organization that everyone would fear and respect. However, they were ineffectual and somewhat comedic in bickering and arguing with each other.