If you don’t know what beholders are (the basics) you should read up on the Wikipedia article for them or even better the post at Power Score about them. Suffice to say, beholders are alien things from another dimension/reality that are not only an iconic D&D monster from way, way back, but also the feature monster on the cover of the 5e Monster Manual. They’re a signature character, as much a part of D&D as anything at all can possibly be.
Playing or deploying beholders in combat, RP-ing them so they pop and stand out is hard. Playing, say, gnolls or hobgoblins I found easier–as a DM. There’s a logic there that’s primitive for one and pretty classical for the other. Playing orcs or goblins is harder, but its still tribal and makes a “behavior of tribes” sense. Dragons are tricky to get right because they’re so large (in stature, I mean) that unless you go very BIG with the performance, you’re really just playing a mean person in the body of a dragon.
Beholders and aberrations or intelligent monstrosities are hard because they are–entirely–not human, not primate, not terrestrial. They are an alien mind. The Monster Manual tries to reflect this by emphasizing paranoia and arrogance, but I like my beholders to go to truly scary levels of alien. The less predictable, the better. The more the party feels like they cannot reliably relate to whatever a beholder thinks and feels the terror of what they see as irrationality (but is simply an alien rationality) the better.
So, here are my tips for playing beholders:
1. An Alien Mind Sees Differently, Hears Differently
Seeing through the eye of the beholder, imagining the world from it’s perspective–no creature (especially the party) appears submissive or reasonable or decent or deferrent. No matter what the party is doing or saying, imagine that the beholder sees them performing the most heinous acts and perverse behaviors possible. Slicing their own flesh like Firefly Reavers… vomiting in each others’ mouths in some orgasmic display of alpha behavior… all their conversation (whether the beholder understands it or not) is coded tactics for torturing and destroying the beholder, themselves… everyone, every creature is a repulsive, dangerous, prepared, violent, and horrific thing here to murder you. When you layer that onto a beholder in your dungeon, it gives flavor and a deep well of things they can say or scream in fear and disgust at the party (for those speaking Undercommon) that should make the party absolutely uncomfortable with an alien mind.
Imagine them running up on one and it absolutely 100% FREAKS OUT about “DON’T TOUCH ME!” and horrific terrified disgust at the party. Nothing to reason with. It sees horrors and treachery everywhere. Remember, they’re all unclean and a moral threat to your existence.
2. An Alien Mind Does Not Perceive Space Like You
My favorite is to challenge the players on their idea of what a dungeon is using beholders. Imagine… you have omnidirectional, always-on floating. Sleeping, wake, doesn’t matter. Can’t be “knocked down. Your home dimension is a mad place of unreality. Make the dungeon that holds a beholder (or at least the part with them) completely unrelated to conventional directions. I like to imagine a circulatory system or an ant colony. Tunnels that rise and twist and change and fall and extend and stop in many directions with no respect to needing a floor.
While the party is climbing upward through a sloping tunnel 40ft wide with random branching tunnels off of it every 50 to 100 ft or so… something floating from their lower left out of one tunnel and blasting them while they’re all climbing with ropes and standing on outcroppings, only to move into a seemingly random other tunnel… it’s terrifying. It makes the whole experience unpredictable and scary.
3. An Alien Mind Does Not Fight Like You
I like to keep beholders terrifying. Absolutely terrifying. I want the party to see signs of strange life down in the dark… for the Sorcerer to pass an Arcana check whereby I pass him a card that says “beholders” on it… and watch his face drop in a panic and before telling anyone what the card says his characters whispers “Everyone SHUT UP… we need to LEAVE!”
This means, no holds barred. No mercy. No choice. Do your dead level best to murder the party. And, to keep with the theme of unpredictability and inhuman thoughts, I tend to pick one theme for what is attracting the beholder and avoid orienting the beholder in a position that implies it is attacking them (eye stalks and rays have a 120ft range and can shoot in any direction). My target is chosen based on something that would not be guessed, something with no real reference. Like whoever has the most spells-they-can-cast left in the round is the sole target. Or the last person to miss an attack (until a new miss happens). That’s the individual in a given round that to the beholder’s mind is most dangerous or most worth of eradication. And I mean eradication.
How I keep this up is to make the antimagic field from its main eye invisible (except for True Sight). Antimagic fields are invisible anyway, by the rules, but DM’s often draw the cone so players know what’s up. I don’t. I make a small depiction of the battle space and every round I orient the cone on the mini-map appropriately using a notecard to hide it from players–revealing “was it on” and “what position is was in” at the start of the beholder’s turn. This means that the casters and magic-users in the party find out most rounds whether they’re in the field’s range or not when they attempt a spell. Now, I don’t eat their action–if they can’t cast then they just can’t cast–I use it to keep them offbalance. Keep them constantly unsure of what they can do until it’s their turn and they must choose what. The tension is palpable.
As far as eyes go… I go all out. Concentrate fire with Legendary Actions on the target(s) and stopping only when Petrified or destroyed. Nothing less. Murder one party member, the rest have to choose to stay or run. It is a hard choice, I promise and given my dungeon setups for beholders, running is usually best accomplished by taking falling damage and dropping through and running around tunnels and hiding.
If I’m using the beholder as a larger device in the story, they devastate one person in the party (perhaps kill, perhaps just petrify) and flee up (it’s hard to chase upward) through tunnels and use their Lair to keep an eye on the party. There will be no rest–it attacks when they stay still long enough. Hit and run. I usually let it heal half its lost hit points during its “wait” time (if it’s an hour or two in game) and have it come back in from a unpredictable direction (perhaps below using its disintegration ray to come up through the floor in an instant) to target someone else and devestate THEM.
Beholders are slow, so direction of attack and withdraw is important to be mindful of. 40 ft. up into darkness is “farther away” than 40ft horizontally where things can target and chase it.
All in all, as a signature monster with legendary actions, treat your beholders as a pure moment to scare the players and make their butts clench mighty tight.
If you can bring together the elements of addressing how they see the world (their disgust at other living things, their hatreds and fears), how they move through it (really play with gravity and directions in less predictable ways), and have them represent a truly lethal encounter (even if within moderate CR range, you can play dangerous with it by focusing attacks and optimizing its combat potential), then you’re on your way to players having memorable beholder encounters and start building your reputation with them.