What's the best way to roleplay a Beholder shooting with its many rays at a Major Image illusion?

Question

We're running the Dragon Heist campaign and the party is facing Xanathar.

The bard came up with an idea to distract the Beholder with the Major Image spell.

The description of the Major Image spell says:

Physical interaction with the image reveals it to be an illusion, because things can pass through it.

But what about a magical interaction from a Beholder's ray?

1. Beholder's rays are definitely not a "physical interaction", but should the Beholder notice that his rays are just going "through" the target, not affecting it at all?
2. Or he shouldn't be able to see that the ray went through the target because he can't see what's behind the target?
3. Or the Beholder should notice the strange fact that the target hasn't even flinched when it was attacked by a super deadly and scary ray?
4. Or maybe the "experienced" Beholder would know, that sometimes such stuff happens when his enemies "succeed with the saving throw" 😁 or when the enemies are somehow immune to some of his rays?
5. So if the Beholder is targeting a well-known seasoned warrior, it should not surprise him that his rays are not affecting the character, like, at all?
6. So the Beholder should continue to waste his rays on the illusion indefinitely? Or when should it stop?
7. Or maybe it IS actually a viable tactic to indefinitely fool the Beholder with the Major Image illusion because the spell description says: [the illusion] seems completely real, including sounds, smells, .... Would seems completely real mean that the illusion will be "flinching" when attacked with a ray? Isn't that "real" for an illusion?
8. Or maybe the bard controlling the illusion can declare the following action: jump out the cover, start a "conversation" with Xanathar with some nasty insults about his pet fish, get 3 deadly rays as the "response" to that if the Beholder shoots and then "carry on this conversation" by "dodging and taking a hit" and then jumping back behind the cover (because the spell description clearly says that "you can cause the illusion to ... carry on a conversation, for example."). I'd define "carry on a conversation" as "IF the other guy would say A to my illusion, THEN my illusion would respond with B". How is it different from "IF the Beholder would shoot at my illusion, THEN my illusion would flinch and show off as if has hurt me badly"? If "carry on a conversation" is clearly allowed, doesn't it mean that while executing your action to control the illusion you CAN REACT to the changing environment if you've anticipated such events? (changing environment being the Beholder shooting the rays at the illusion)

As a beginner DM, I obviously know that it's an illusion. The party knows that I know, and I don't want to make it look like the Beholder got an "unfair advantage" from the DM because of that. So I have to roleplay the Beholder pretending that he sees a real enemy and should be trying to attack him. On the other hand, I really want to make this encounter balanced and running for 5 turns all over the battlefield, "absorbing" 100% of the Beholder's rays, should probably not be a viable tactic... Where is the happy medium?

---

UPDATE: Some important context here (on why the Beholder will probably NOT "just point the Antimagic Ray on the target and reveal that it's an illusion"):

Sure, the Beholder has his Antimagic Cone and pointing it at the illusion would've exposed the illusion immediately. But if we'd take a birds-eye look at the whole battlefield, then the most logical tactic for a Beholder would be this:

• Point the Antimagic Cone at the most dangerous spellcasters or on the main target (the wizard carrying the artifact in case of our encounter)
• While the spellcasters are "disabled" and are basically "sitting ducks" - approach them with some melee-fighting minions
• Set a readied action: "if anyone of the main party members shows up anywhere outside the Antimagic Cone - shoot him with the 3 deadly rays"

I don't know how Beholder encounters usually happen for others, but I suspect that this 3-item strategy sounds quite logical for a Beholder in most of the cases. Anyway, this is how it looked for our encounter:

So, the party learned the hard way that the Beholder is smart! He knows every "VIP character" on the opposing side and will shoot anyone of those VIPs with his readied action as soon as any of those folks would show up outside the Antimagic Field. And three rays being shot at one PC at the same time would most likely mean this PC is DEAD in one turn. I'd say that for any party members and NPCs, "being that first guy to take 3 rays for the party" would certainly be suicidal and "out of character". But someone has to be that guy who will take that first alfa-strike on him? Why not make the illusion to take those hits and THEN make a move?

So, the party came up with a plan: create an illusion of one of the VIP characters from a safe place and make it look like one of the VIP characters is stepping outside of the Antimagic Cone and provoking Xanathar to waste his "alfa strike" on this turn and then hiding behind cover at the end of the turn so it will not be revealed in case Xanathar moves his Antimagic cone towards the illusion. So that other party members can safely step outside of the cover in the zone without the antimagic field and safely throw their spells combos against Xanathar without being afraid to get 3 rays right in the face... Then, repeat the same on the next 1-2 turns: the party's combo starts with the illusion showing up and provoking Xanathar to strike it with his 3 rays.

To sum up: there are strong tactical reasons for Beholder NOT to move the Antimagic Cone away from the main targets with their scary spells (Team A) onto a single annoying illusion, but to just "kill it" with rays. Especially if the illusion is a non-spellcaster, no reason to waste Antimagic Cone on a barbarian or a monk... But will the Beholder still be wasting all his alfa-strikes on all of the next turns on the same target?

Answer 1

Xanathar is not stupid

The rays are not physical interaction, and therefore shooting a ray at the illusion will not automatically reveal it.

However, normally creatures are not unaffected by the rays, at least not being hit my multiple rays. This should be the Beholder's experience, and if the illusion does not behave like that, the Beholder — who does have a very high intelligence — should become suspicious, just like the players would become suspicious that something might be an illusion, if it does not react to their actions in a way they would expect.

The full section on discerning the image as an illusion in Major image says:

Physical interaction with the image reveals it to be an illusion, because things can pass through it. A creature that uses its action to examine the image can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC.

So once Xanathar becomes suspicious, he can try to use his action to discern the illusion without physically interacting with it.

Or, if it is within its range, he can point his antimagic cone at it — that suppresses it, which would be a dead giveaway it is a magic effect, and even if it's still not a physical interaction revealing its nature, you either could count it that way or at least have Xanathar decide something fishy is going on, and ignore it. (The players might be able to avoid the beholder reaching the illusion with its ray depending on positioning, as the beholder is pretty slow moving, so they can move the image out of reach).

The rules do not say if the rays are visible. Judging from the disintegrate spell, which creates a green ray, they might be (they are also depicted as visible in WotC published material like the Magic the Gathering Baleful Beholder card, but that may be artistic license). Even if they are, the illusion that is opaque would block the beholder from seeing that they go through.

You have to decide on where the "happy medium" lies yourself. I probably would have Xanathar suspect something after a couple rounds of wasted effort, or roll Intelligence checks against a set DC for him after each round to determine when he catches up.

---

UPDATE: Addressing the specific situation and questions posed in your update and comments.

Firstly, I assume based on your comments, Xanathar will not ascend high to cover the battlefield in an antimagic zone with his main eye ray.

Secondly, to be a bit nitpicky, getting hit by three eye ray does not most likely mean the PC is dead. Only the enervation, petrification and disintegrate rays actually damage or perpetually harm the target. Enervation should be well survivable by PCs who can upcast spells to 6th level (Major Image). That leaves 2 out of 10 possibly deadly rays. At three rays, that is a less than 50% chance to be hit by one of these rays. I however can sympathize with the PCs not wanting to test these odds.

Role-playing Xanathar

Now, I think your strategy for Xanathar is reasonable enough -- keep the main spellcasters in the antimagic ray, beat them up with goons, and snuff out other opponents with his eye rays (or send extra goons).

Instead of continuing to craft an optimal plan how Xanathar could counter all the PCs' plans, my recommendation is: don't do that. Yes, Xanathar is pretty smart, but in spite of what he thinks of himself, he is no super-genius. He has Intelligence 17. The party wizard likely has more. And unlike you here, he does not have a kaggle of armchair tacticians at his command who can spend hours of their spare time to craft the perfect foil to the PCs' plan. He has 6 seconds to come up with something. He's not all-knowing and perfect.

Ignoring that is bad style, by at least in this DM's view. For the PCs, it is much more fun and rewarding if their carfully laid plans sometimes work out. If the bad guys are fallible too. On the other hand, in my experience it a safe way to kill their enjoyment is when th DM uses out-of-game knowledge of the PCs abilities and the situation to let the monsters make tactically optimal decisions they normally could not make. That would be poor, adversarial DMing, it breaks immersion, and it sucks the fun right out.

First round

At this point, Xanathar does not know about the hidden additional casters. What I wrote in my original answer still holds. He has no reason to not fry the illusion with readied rays when it shows up. Then, he may become very suspicous if that does nothing. He also has up to three legendary actions with eye rays, which he will be able to use either against the illusion or the PCs when they move out to attack him, (unless they manage to get back behind cover before the end of their turn; if they have normal 30 feet speed, this does not look likely for at least the one further in the back).

Follow-on rounds

I however wouldn't bank on this decoy trick working for the following rounds. If Xanathar tried frying the illusion once and it did not work, at his Intelligence, he should try something else. Maybe send some goons. Maybe ready his triple ray against the casters who came from behind the main building that just did cast spells to harm him. Maybe shoot legendary rays against the boxes behind which the illusion "hides", if there is no other target, to move them with telekinesis or disintegrate them. This way, their decoy worked, and they got something out of it, but it is not a free ticket that lets Xanathar look like a dumb fool.

Regarding having the illusion insult Xanathar, and carry on a conversation to attract his attention to the illusion, I think it could work. It would cost the caster's action, so no other spell attack by them. The question also is if Xanathar evaluates this as important in the current situation or gets emotional about his fish. You could have the PCs roll on Deception or Intimidation against his Insight, to see if he falls for it, and if they succeed, they get one more round of him chasing the wrong target out of it.

Answer 2

There are no secret rules

The beholder's eye rays are not physical interaction so they don't reveal the illusion.

You have assumed that the eye rays are perceptible - the rules on beholders don't actually say this.

This won't work for long

While the eye rays won't reveal the illusion, the Antimagic Cone will suppress it - which is a dead giveaway for anything smarter than a frog - beholders are much smarter than that. Once the cone is removed the illusion will return but the beholder can safely ignore it.

Answer 3

This should basically never happen.

In order to use its eye rays on something, a beholder must be within 120 feet. The range of its antimagic cone is 150 feet. The range of a major image is limited to 120 feet of the caster. A beholder also can't fire its eyes rays through its own magic cone. This creates the following arrangement.

1: Your beholder is within 120 feet of the illusion, and is pointing its eye cone at the illusion. The illusion stops being a target until it averts its eye cone, and it can't fire its eye rays at the illusion.

2: The beholder is within 150 feet of the illusion, but greater than 120 feet away. It can't target the illusion at this range.

3: The beholder is within 120 feet of the illusion and pointing its antimagic eye in a different direction that also doesn't include the well-known mage or the bard for whatever reason. As soon as the major image is created and the bard ends its turn, Xanathar has a legendary action eye-ray to throw at the Major Image, and if your mage used their action on anything else, they aren't using an action to identify the magic of the eye ray, which means they don't know which effect to simulate the illusion being hit by. (It doesn't make sense for the illusion to 'turn to stone' from a petrify ray if Xanathar used a Death Ray).

4: Why isn't the beholder pointing its eye cone at the mage or bard? The moment its antimagic cone touches them, they lose their ability to interact with the spell, which by the way, reads:

As long as you are within range of the illusion, you can use your action to cause the image to move to any other spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image. For example, if you create an image of a creature and move it, you can alter the image so that it appears to be walking. Similarly, you can cause the illusion to make different sounds at different times, even making it carry on a conversation, for example

The image can only be caused to move with the caster's action, and the caster can only alter the appearance of the image during its movement (caused by the bard's action.) It's not a Programmed Illusion, and any attack that happens to the Major Image outside caster's turn automatically lacks an appropriate visible reaction to the attack, unless the caster-

A: Isn't in the antimagic cone B: Used their action to Ready Action -> Move the illusion appropriately when it's attacked.

5: If the party is hiding and creates the Major Image within range of the beholder, the beholder has nothing but the illusion to focus its antimagic eye on- and will thus immediately know not only that it's an illusion, but that something had to create the illusion in the first place.

6: While you can make a Major Image move, carry on a conversation, and do other things that would make it seem real, like 'flinch' and 'bleed' when being struck, it's important to note that this paragraph of detail is directly conditioned on the caster using their action for "making its movements appear natural for the image." In your example, the bard created the major image and as part of casting the spell has directed the illusion to insult Xanathar. The illusion, within its currently occupied space <it only moves using the caster's action> will sit there and insult Xanathar, colorfully, perhaps while hiding behind cover. You as the DM might allow the caster to condition 'flinching when struck in response' as a reasonable reaction to insulting a beholder, but this is still a very non-specific reaction to something like a petrify ray or a sleep ray, and it's highly unlikely the bard predicted that as well on a 1 in 10 chance.

7: Your specific case

A: The party is smart, I'll give you that- but Xanathar is probably still smarter than everyone but the wizard, and who knows how many years of experience he has?

Let us start with one very important fact- this encounter appears to be outside, meaning no ceiling.

B: Let us follow this up with an interesting tidbit about Xanathar's psychology. It's the supreme being of the universe. Its natural place in every conceivable way is above all other beings (reflected by the fact that its combination of Hover and Prone immunity make it basically impossible to force it closer to the ground without grappling with it- problematic at the outset unless your party has nonmagical methods of flight).

Xanathar will not simply be outfoxed. No, no, it is clearly superior, and it will do the outfoxing. These pesky humanoids think they've cleverly forced it into a position where it has to choose which of them it covers with its eye cone, but they underestimate its power.

Xanathar has eternity to kill these fools. It will succeed, but in the process it must be careful not to allow lesser beings to outmaneuver it, or it would be the laughingstock of the multiverse, even after its lessers inevitably meet their doom. Its reputation might take centuries to recover. No! This is absolutely unacceptable. Xanathar will take its time killing them, and ensure it does so neatly, without ever sacrificing his naturally superior position.

C: Okay, now that this disturbing trip down Megalomania Lane has ended, let's talk 3d combat and cones. A cone's width at its maximum length is defined as equal to the length of its sides on a grid. Translated to a proper cone shape, this means that a cone of any length pointed straight down at the ground from in the air is exactly as wide in diameter on the ground as it is high from the ground, so long as your altitude doesn't exceed the range of your cone. Xanathar's fly speed is 20 feet, which means if he uses his action he can gain 40 feet in altitude against characters with non-native fly speeds. We'll get into the consequences of trying to chase him into the air later.

We're almost ready to play this out, but first, let's talk about Xanathar's preferred readied action, because here's where things get really fun. The prospect of eating three beholder rays is certainly intimidating- but it's also a little predictable, particularly if your whole party is expecting it and concocting this entire plan around it. However, again, this cannot be emphasized enough, without using its action, Xanathar is still entitled to 3 attacks per round of initiative in this scenario with a party of 4. Rather than ascending 40 feet, it can ascend 20 feet, and then ready its action to move so that when a caster not in its cone moves its mouth, it's now in the cone.

8: How the alternative plays out

Based on your image above, there is no scenario in which I can imagine surprise for either side at this point in time. My initial thought looking at this scene is that if there was a surprise round, it must have played out when the major image was created - otherwise at the end of the creator's round, when the taunting image appeared, without Xanathar having to use his action, it can instantly hit the MI with a legendary action, which may immediately reveal the ruse. Round 1 has presumably already happened, but I'm going to start with 'round 1' meaning from the point you've got drawn up.

Round 1

I'm going to assume it's Xanathar's turn. Xanathar adjusts its eye cone to make sure the mage is still well within and uses its movement to ascend 20 feet directly to the northeast - remember, diagonal movement exists in 3D - and sends its 3 closest melee minions at the Major Image, while the 1 (+2 hidden) on the left are directed to the mage and his buddy. It uses his action to ready an action- if it sees a known spellcaster take ANY action, including move their lips, take the dash action. You don't have to activate this the first time it triggers, you can activate it any time the trigger is met for the round.

Now it doesn't matter who goes next, Xanathar takes and maintains the advantage. If the mage goes, they can't cast inside the cone. If they move, Xanathar can ascend a further 20 feet, widening the amount of ground around mage covered by the cone, and moving as needed horizontally since it can only reorient its cone on its own turn. If the mage doesn't move and force Xanathar to expend his readied action on movement, the mage has made a grievous tactical mistake. Oh, and for giggles, Xanathar throws a random eye ray at the impudent creature yelling insults (Legendary action 1/3). "Hmm, that's weird, it flinched from my sleep/petrify/fear/charm/paralyze/slow ray..."

Xanathar has removed itself from the melee equation entirely. What's more, it has a passive perception of 22 and is likely to see the party members hiding on the eastern side of the map from its increasing altitude, and if they break cover before that happens to establish LoS for an attack, they're likely to be spotted.

With how far the party is spread out, on round 1, Xanathar won't be able to cover them all with the antimagic cone. However, the two uncovered party members on the right probably can't put Xanathar down in a single round, and it's possible they won't want to give up their position right away on round one when Xanathar's response is to move upwards and not unload attacks. Indecision will cost them- they've already lost the edge they were planning for. Even if they don't attack or reveal themselves, Xanathar still gets has 2 legendary actions to burn. (17 intelligence- hmm, it flinched from a sleep ray and a petrify ray AND a death ray the exact same way...?)

The minions close to engage the party and Major Image.

Round 2

Xanathar angles its eye cone further down towards the ground, and moves another 20 feet up and north east. At this point its moved 60 feet up and northeast, placing it 60 feet directly above the square two squares north of the crates the Major Image is using for 'cover.' At 150 feet of cone with a downward angle towards the mage and his friend who are being closed in on by melee minions, and without magical movement, there is no longer any movement the mage can make to get out of the cone that will allow him to maintain line of sight to Xanathar. Xanathar now also has a clear aerial and horizontal line of sight to the bard and his friend behind the wall.

Xanathar's eye rays did nothing appropriate to the major image - it's no longer wasting valuable energy firing at it, especially since it has three goons ready to tear it up in melee- the first swing of which will immediately reveal its illusory nature. It readies its action to move.

At this point Xanathar himself is safe. As soon as the two actual party members underneath him reveal himself, it can move to use the buildings to break LoS while ascending further, putting it 80 feet above and completely to one side of the whole party, which puts it in a position to use its antimagic cone for cover from attackers beneath it. If they don't reveal themselves, it still has no reason to continue engaging the major image, and eventually it will ascend high enough to include the entire battlefield, including the image and the potentially invisible party members, in its cone.

Everything After

In either case once this scenario is reached, the party has the option of attempting to exchange nonmagical ranged weapon attacks with Xanathar's total of 6 eye rays per round while it harasses them from max range in the sky. The party could attempt to fly up to engage Xanathar, but if your party is trying to fly up to a beholder in the sky it should be giggling while the stupid mortals fall- painfully- over and over, unless someone in your party has a naturally flying mount not bound to them by a spell.

Even if none of this works according to Xanathar's plan and the party is somehow outpacing Xanathar's damage, unless they can punch through enough damage to stop him from flying away without trying to chase him through the air, his survival is basically guaranteed, and he's in a good spot to guarantee a victory, as well.