Hmm I am thinking of having the beholder be asleep and the risk is that the villain will try to wake it. so theres a chance the players will not have to face the slumbering beholder, but if it does waken i could establish its power quickly by having it target the NPC ( a necromancer elf and brother of the wizard Awlin ) and a few of the other heroes allies showing off its incredible power before turning its rays on the players giving them plenty of time to either formulate a plan or get out. Or even if it wakes have it not decide to kill them immediatly, have it try to dominate or humiliate them.
Beholders are fully aware of their surroundings even while they "sleep".
I will say this, I don't think the beholder would be interested in immediately killing the party. It may want to just escape so it can rebuild its powerbase. They are mastermind types, face to face confrontation is the very last thing a beholder wants to deal with. But if it does it should have a plan to deal with the party without its direct involvement.
A Beholder wouldn't escape; not immediately. Every Beholder believes all other creatures (including other Beholders) are inferior to them, so it has no reason to think it's going to lose. Also, because every Beholder thinks the world revolves around them, they're going to interpret the party's presence as proof of a conspiracy to kill them. If they're aware of other Beholders, they're going to think one of them's finally made a move. A Beholder would only flee after it becomes apparent it can't win the fight.
Not only does it know it's superior to the party, it's not going to fight fair. The first thing it's going to do is fly out of 60' darkvision range and if anyone follows with magical flight, it'll open its antimagic cone on them. If fighting fails, it'll have a backup plan. Maybe it built its boss room under an artificial lake or dam and it disintegrates a hole in the ceiling so it floods. Maybe it's rigged the ceiling to collapse when it disintegrates the support pillars. Maybe there's a pit under the floor and it disintegrates the floor out from under your feet. After that, it would try to escape.
Unless the beholder was calling everyone to its location to free it from its cage. The beholder summoned the dark elves from accross the sea through its maddening dreams and potentially the party as well, it will percieve there presence here as apart of a deeper plan to gain its freedom.
Is the anti-magic cone visible to the PC's?
If you want sugar coating, go buy a dessert....
We just fought a Beholder in a one shot.
I just wanted to add that it is a quite satisfying opponent.
As most of their attacks are resolved with saving throws, the majority of the dice rolling will be the players.
This feels like they have agency and are not just keeping track of hit points.
More Interesting Lock Picking Rules
Is the anti-magic cone visible to the PC's?
If you want sugar coating, go buy a dessert....
I've never seen or heard of it played as a visible effect.
Then again, Beholders are imagination transformed into reality, so if you wanted it to be, it can.
We always played it as an invisible field, only noticeable when your magic failed.
More Interesting Lock Picking Rules