This is my first time posting so the post may be a little scruffy. I'm running a highly edited game of GoS and I would like my main BBG to be a Beholder cult leader since we'll hopefully go to level 20. It is made up of nobles who want to increase their status. In my world the Beholders waged war on an ancient empire twice. They wish to rule the surface world. Please help me flesh the cult out.

Check out Xanathar’s Thieves Guild in the forgotten realms wiki or Waterdeep Dragonheist for inspiration. Basically runs like a beholder cult. You can take what you like and just ignore what you don’t.

“Members of the guild were trained in stealth, discretion, and tactics with fighters, rogues, sorcerers, wizards, and expertswhich comprised most of its numbers. An ever-growing organization, it utilized multiple mouthpieces to protect and conceal the identity of the Xanathar. To join, a member had to prove their ruthlessness, obedience, and loyalty by committing a serious crime - like killing or selling a friend or family member into slavery, as well as paying 300 gp. The typical agent earned dues of 60 gp a month. Support generally came in these ways:

• Any member given an assignment would receive a bodyguard or assistant - such as a bugbear, wererat, kenku, or gazer - with secret instructions to spy on and assassinate the agent in the case failure.
• The guild provided safe refuges - namely cellars in the sewers.

Among the low-ranking guild members, no-one really knew who Xanathar was. Few had an inkling he was a beholder, and fewer still had spoken to him.

Given that the faction was evil at the core, advancement through assassination was a popular "shortcut".”

BEHOLDERS: are Xenophobic and vicious creatures, beholders were quick to attack enemies, including anyone they deemed not "like themselves." Beholders, as a rule, were violent and greedy, hungering for both wealth and power over others. This was made all the more complicated since more than one variety of beholder existed, each believing itself to be the pinnacle of bodily perfection and they viewed other beholders who differed from this image in even the most minute details as loathsome enemies and inferiors.
Beholder minds were divided into two separate entities. Each of these entities thought and acted on its own accord even though it was bound to the same body as the other half of its mind. Neither half of the beholder's mind trusted the other, so they hid a lot from each other, creating a very paranoid relationship. "Sane beholders" were beholders whose minds were not "divided" so to speak. They were still two entities within the beholder, but neither hid anything from the other, making a less paranoid beholder. However, the persona of a "sane beholder" was just as likely to be considered "insane" by any non-beholder. Because there were two entities within a single beholder, that beholder should always be addressed by its full name when in conversation with them, or they would perceive it as speaking to only one of the entities

Hope this helps the creative process for you.

Thanks. Luckily I have Dragonheist.