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?
- 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?
- 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?
- 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?
- 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?
- 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?
- So the Beholder should continue to waste his rays on the illusion indefinitely? Or when should it stop?
- 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?
- 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 alpha-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 "alpha 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 alpha-strikes on all of the next turns on the same target?
Best Answer
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:
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 gaggle 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 carefully 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 the 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 suspicious 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.