Mind Muddler 

Weapon (Whip), rare (requires attunement by a creature which can speak Deep Speech or telepathically)

Mind muddlers were designed by Illithids as a tool of torture which would soften both the flesh and the mind, making the target more susceptible to their psionic intrusion. The whip is made from a flesh-like material which looks akin to a long, thin tentacle. 

This whip acts as a magic weapon granting a +1 bonus to attack and damage rolls made with it. When you hit a creature with an attack made with this weapon, it must subtract 1d4 from the next saving throw it makes before the end of your next turn. This effect does not stack if a creature is hit by more than one attack made with this whip before it makes its next saving throw. 

This item has 4 charges and regains 1d4 charges each dawn. Once per turn, when you hit a creature which has a brain with an attack made with this weapon, you can expend one charge and force the target to make a DC 13 Intelligence saving throw. On a failed save, you can reduce either the target’s Intelligence score or Wisdom score by 1d4. This reduction lasts until the creature finishes a short or long rest. If this would reduce either of the target’s scores to 0, the creature dies. 

Monster Variant: Mind Flayer Scourger

You can modify the Mind Flayer stat block from the Monster Manual, adding the Multiattack feature to its list of actions which allows the Mind Flayer to make up to three attacks with the Mind Muddler (+4 to hit, 1d4+2 damage on a hit). If the Mind Flayer is within 5 feet of a target, it can replace one of these attacks with an attack made using its Tentacles.

Array of Scores: 15, 14, 13, 12, 10, 8 (standard array)

I have chosen to use the standard array to give an "average" view of how this build might look. 

Race: For this build, I have decided to use a Mark of Hospitality Halfling. This comes with a few useful things

• +2 Dex, +1 Cha
• Lucky (free reroll on “1”s for most d20 rolls)
• Brave (advantage on saves against being frightened)
• Halfling Nimbleness (can move through bigger creatures’ spaces)
• Ever Hospitable (+1d4 to Persuasion checks and those made with certain tools)
• Innkeeper’s Magic (Additional spells) 
• Can speak Common and Halfling

Ability Score Distribution: 

Strength: 10

Dexterity: 12(+2)

Constitution: 13

Intelligence: 14

Wisdom: 8

Charisma: 15(+1)

Charisma is our main stat for this build, as we want to take the majority of our levels in Sorcerer. Intelligence is 14 for the sake of multiclassing. Dexterity is given the 12 because it will be offset by the +2 bonus from our race. Constitution gets an odd number with 13, but we will bump this up later. 

Background: For our background, I have chosen the Quandrix Student, which gives us access to several more spells like our subrace, but most importantly gives us the Strixhaven Initiate feat, allowing us to cast Guidance and take Silvery Barbs as our 1st level spell. We can already get Silvery Barbs as a Sorcerer, but this gives us a 1/long rest free casting and leaves our spells chosen from the class more open.  

Level 1: Sorcerer 1 - Clockwork Soul

We will begin as a sorcerer with the Clockwork Soul sorcerous origin. Our initial proficiencies give us Charisma and Constitution saving throws as well as several skills. Which skills are chosen are not of much importance, but avoid Arcana as you get this from your background. 

Our subclass gives us even more spells as well as Restore Balance. This allows us to remove advantage or disadvantage from a d20 roll we can see within 60 feet using our reaction. 

Level 2: Sorcerer 2

Here we get Font of Magic, giving us sorcery points but not much use for them yet. 

Level 3: Sorcerer 3

Here we get Metamagic. There are many good choices that can be made, but to play into the theme of “fate control” we are going to take Empowered Spell (allowing us to reroll damage dice for our spells) and Heightened Spell (to impose disadvantage on a saving throw). 

Level 4: Sorcerer 4

We get our first Ability Score Improvement, which we are going to use to take the feat Second Chance from Xanathar’s Guide to Everything, which is unique to the Halfling race. This feat will allow us to bump our Constitution score from a 13 to a 14, improving our health and concentration saving throws, as well as the ability to force a creature to reroll an attack roll once per combat encounter or rest. 

We will also take Sorcerous Versatility from Tasha’s Cauldron of Everything to allow us easily change out metamagic and cantrips. 

Level 5: Wizard 1

As we start into Tier 2, we are going to multiclass into Wizard. Since we are multiclassing two full casters, this should not greatly delay our spell slot progression; however, we will end up with spell slots higher than the level of spells we can prepare/learn owing to multiclassing. Because the wizard spell’s will use our lower Intelligence modifier, we will focus on taking rituals and other spells which will not require us to make an attack roll or an enemy to make a saving throw (such as Magic Missile or Shield). 

Level 6: Wizard 2 - School of Divination

Now we get access to our subclass for wizard, which we will choose the School of Divination so that we have access to the Portent feature, allowing us to roll two d20s every day and replace rolls with those values, with no action required. 

{Alternative Choice: Choosing the Chronurgy subclass for its Chronal Shift feature would also be a good choice for controlling fate; however, I have gone with Divination for this build as Chronal Shift requires use of your reaction, and there are alot of other features that will already be competing for its place. }

Level 7: Sorcerer 5

Grabbing 2 levels from Wizard was all we really needed, which gives us access to Portent and a handful of more spells. So now we move back to progressing in Sorcerer, which we will follow for the remainder of this build. 

Level 8: Sorcerer 6

Level 6 in sorcerer grants us access to the Bastion of Law feature from our subclass. This allows us to spend sorcery points once per long rest to create a magical ward which negates damage. 

Level 9: Sorcerer 7

Level 10: Sorcerer 8

As we finish Tier 2, we will grab our second Ability Score Improvement. We will keep this one simple and use it to buff our Charisma modifier from 16 to 18, which is the maximum it will reach for this build. 

Level 11: Sorcerer 9

Level 12: Sorcerer 10

Pick up another Metamagic option of your choice. To stay on theme, I would recommend Careful spell as a way of “altering fate” so that your spell conveniently misses your allies. 

Level 13: Sorcerer 11

Level 14: Sorcerer 12

Now we get our third Ability Score Improvement and we are going to go back to grabbing feats. This time, we will take Bountiful Luck, another feat from Xanathar’s Guide to Everything which allows us to extend our ability to reroll “1”s on most d20 rolls to our allies. 

Level 15: Sorcerer 13

Level 16: Sorcerer 14

Here we get our next subclass feature from our sorcerous origin, Trance of Order. This grants us the ability to enter a trance for 1 minute as a bonus action. During this trance, attack rolls against us cannot have advantage and for most d20 rolls we can treat a value of 9 or lower as a 10 on the d20. 

Level 17: Sorcerer 15

Level 18: Sorcerer 16

Because we multiclassed, we start Tier 4 getting our final Ability Score Increase, which we will use to get the tried and true Lucky feat from the Player’s Handbook, giving us three luck points by which to affect the outcome of most of our d20 rolls as well as attacks made by enemies. These points can be spent to affect the rolls without using your reaction, so they can be used on top of things like your Restore Balance feature. 

Level 19: Sorcerer 17

We get our final Metamagic option. Since we already have the thematically-fitting options, I am going to pick Quickened Spell as it is a reliable metamagic. 

Level 20: Sorcerer 18

We finish this build getting our capstone ability from our sorcerous origin, Clockwork Cavalcade. As an action, we can restore up to 100 hit points across several creatures, undo damage to objects, or end spell effects of 6th level or lower. 

I will not delve into the entirety of this build’s spell kit (which is quite large between the extra spells/expanded spell list from our subrace, background, subclass, and multiclassing). I do, however, want to take the time to highlight a handful of spells that will fit into this build.

Cantrips

From Quandrix Student and the Strixhaven Initiate feat we get access to Guidance, which allows us to further twist fate to our liking by allowing us to add 1d4 to ability checks for ourselves and allies. We also grabbed Mind Sliver, allowing us to impose a 1d4 penalty on enemy saving throws. 

From our Wizard multiclass, we have picked up Control Flames, Mold Earth, and Shape Water which can all be flavored as us exerting our will to alter the world around us. 

1st level Spells

From Quandrix Student and the Strixhaven Initiate feat we have access to the Silvery Barbs spell, allowing us to use our reaction to impose disadvantage on a d20 roll and give advantage to another creature. 

We have also grabbed several other reaction spells (mostly from Wizard) to help keep our character safe (Absorb Elements, Feather Fall, and Shield). 

3rd level Spells

We have picked Counterspell to give us some control over which spells our enemies are allowed to cast. 

5th level Spells

With the addition of Synaptic Static, we now have another route by which to influence certain d20 rolls of our enemies, with targets needing to subtract a d6 from their attack rolls and ability checks. 

9th level Spells

Finally, if our goal is to control fate and break the 4th wall, the Wish spell is a must have

--------

Knowing that our character has Silvery Barbs and several spell slots to use it with, we have also filled out our spell list with several “save-or-suck” spells that could benefit from having a creature be forced to reroll its saving throw (Disintegrate, Dominate Monster, etc.)

As a spellcaster, outside of the handful of spells we pick to play into the theme, we have the freedom to pick whatever spells we want. Additionally, with our subrace, background feat, and multiclass we have access to more spells and more spells to choose from, allowing the character about as much versatility as a Sorcerer might normally have, if not more. 
The main focus of this build is, of course, dice manipulation. So what can we do? I have outline the possible spells/features we possess which modify rolls in some way based on the type of action they require

In terms of how frequently you can use these:

Racial Traits: Ever Hospitable and Lucky have no limitation on how often they can be used. 

Class Features: At level 20, Restore Balance can be used 6 times per long rest, or roughly once or twice per encounter. Trance of Order is limited to once per long rest unless you are willing to allocate Sorcery Points to use it again. Similarly any Metamagic we use are limited by our sorcery points. Finally, Portent can be used twice per day. 

Feats: Lucky can be used three times per day. Second Chance can be used once per short or long rest. Bountiful Luck has no limitation on how often it can be used beyond requiring our reaction and limiting our usage of Lucky (racial trait)/

Spells: These are limited by our spell slots, with the exception of cantrips like Mind Sliver and Guidance. Furthermore, we get one extra free use of Silvery Barbs per day from our background feat. 

Example Scenarios

1. You want to attempt a Persuasion check to convince someone to give you a discount on a sale. If you had cast Guidance, you roll 1d20 + 4 + 1d4 (Ever Hospitable) + 1d4 (Guidance) and since you can reroll 1s using Lucky, the minimum you can score is an 8 and the maximum is 32. If Trance of Order is active, the minimum you can roll becomes 16. 
2. A creature targets you with an attack roll, you can: force a reroll with Silvery Barbs or a luck point or Second Chance, replace the roll with one of your Portents, or cast a spell like Shield to bump your AC up. If the creature had been previously targeted by your Synaptic Static spell, it would also be subtracting 1d6 from its attack roll. 
3. You really want a monster to fail its saving throw against your Dominate Monster spell. If you had previously hit it with Mind Sliver, the monster subtracts 1d4 from its saving throw. Regardless of this modification, if the target succeeds on its saving throw you can cast Silvery Barbs as a reaction to force it to reroll its save, potentially causing it to fail. If it still succeeds you could then rely on the use of one of your low Portent rolls (as this requires no action).
4. You risk failing a saving throw. If you roll a 1, you can use Lucky (racial trait) to reroll. If you roll something above a 1 and still fail, you can use a luck point to reroll. If you still fail, you could potentially replace the roll with a high roll from your Portent dice. If Trance of Order is active, the minimum you could roll is a 10.

Quote from Semako >>

Quote from ThorukDuckSlayer >>

Neverending nightmares is a cool concept, I might have gone for an increasing DC (starting at 12 and increasing by 2 for each long rest), with a success allowing you to rest a bit (half health regained, and half spell slots restored), to give it a more creeping-up effect.

First, thank you a lot for your feedback!! I am glad you like this statblock, and I hope you will have some sweet dreams tonight... :-D

I will change that, thank you for the suggestion. I will start the DC at 10 and increase it by 5 for each attempted rest, which each fail resulting in a long-term madness. After all, a character who rests for multiple days in such an area deserves to get a multitude of madnesses...

Cone of Draconic Wrath is very nice, and I like how it allows for a range of beholdragons. The mention of secondary effects makes me wonder if this needs to clarify that it's the primary breath weapon (EG not the repulsion breath or enervating breath).

Yes, it indeed means the primary breath weapon. With the secondary effects I was thinking of gem dragons and other non-metallic/non-chromatic ones that have breath weapons with secondary effects (also, I will soon have Sardior's statblock finished!). I am not sure though how I could clarify that in an easily understandable way. On the other hand, you could, if you want, combine the effects of a metallic's primary and secondary breath weapon into the Cone of Dragon's Wrath effect.

Eyestalks, I love that you can cut the eyestalks off. That's a good way to make a boss fight more interesting!

Many eyes menace is very creepy, love it.

I am glad you like those effects!

Nightmare ray is going to have players scrambling to get to one another to wake each other up, which is an awesome thing to mix into a fight!

Pain ray is a good way to shut down casters.

I like the Maiming ray, but I would consider having the tentacle be functional, and permanent. Otherwise clever play risks being punished rather than rewarded! "yes, but..." rather than "Actually, no."

Draconic Ray is another nod to the original dragon, helps keep it thematic.

Changed the effect of maiming ray to produce an actually "somewhat functional" tentacle. I am glad you like the other eye rays!

Eyeful presence - Typo (Whie frightened in this way...),  otherwise I like it, especially it invoking Confusion. It is a little concerning that it could literally wipe a party with this one move, as it can do it every turn. Scary is one thing, but perhaps have whoever rolls the lowest be affected by Confusion, rather than everyone? This one ability can drop the party to only having a 20% chance of actually playing this round, and I have to say, that doesn't sound fun for the players!

Typo is fixed already, thanks for pointing it out. Right now it has the confusion effect for everyone who fails and those who fail by 5 instead gain a short-term madness. Not sure how to change it. On one hand, if the party works together, fear can easily be avoided and cured - such as by a heroes' feast spell, a paladin's aura, magic items and other things. On the other hand, you are right, that if the party has bad dice luck, the situation, well, gets dicey fast. In general when I write monster statblocks I always expect that DMs adjust the monster's DC to their party to avoid impossible saving throws, so there should not be a situation where a party is not able to get out of this kind of fear.       
Maybe the way to go is to remove the madness and have the confusion proc when a creature fails by 5 or more? Or remove the confusion and have just the madness effect on a fail by 5 or more? I am also considering lowering a creature's Wisdom stat (similar to Hastur's Reveal Visage, although without paralysis) as this thing is so otherworldly and defies their understanding of the world?

Reality Shattering Landing is an awesome image, I feel it needs split rules for whether you're landed on or next to, I feel like landing on someone should do more damage. There's also need for something to say "after this move, move any creatures which share a space with the beholdragon the minimum distance needed to no longer share a space."

While you are right, I think I am keeping it like that, as it is simpler, more streamlined. It also is how the similar ability for my great wyrms, Bahamut and Tiamat works. Basically, the area around the dragon is what I imagine is covered by the dragon's wings (therefore it is the same radius as the dragon's wing attack legendary action if it has one). I could add that creatures are pushed away, but interpreting it as the creatures are still within the dragon's space and have to move out of it could work too, especailly for bigger dragons where creatures could feasibly be prone beneath the dragon's body.

Antimagic Breath, I wonder if it could have lingering effects on casters - they need to pass a DC23 check with their spellcasting modifier when the ycast a spell or the spell is wasted, and the first time they succeed the effect ends? something like that could have them popping cantrips in a desperate attempt to break out of the antimagic!

Interesting idea, but I think this would be a bit too harsh? It basically shuts them down completely until they make the check or saving throw (I would go for a saving throw here so that they can actually be proficient and beneit from other saving throw bonuses like a paladin's aura, a bless spell and other abilities). Having all spells of 5th level or lower breathed away already is enough in my opinon. It means spells like freedom of movement, [Tooltip Not Found], haste, wall of force, fly, protection from evil and good and many more that would be quite powerful against the beholdragon are gone now, the time used to set them up basically is wasted. Also, the pain ray already cripples casters hard *evil DM laughter*

Tentacle breath, I like the way it works, I feel it could use a little more clarification on restrained creatures moving with the beholdragon - perhaps "after the beholdragon moves, any creatures currently restrained by the tentacle breath are moved towards the dragon until they are 60ft. away. If the beholdragon is flying, they hang 60ft. below it."

I already clarified it a bit more, although I left the part about characters hanging below the dragon out. Not every creature will get caught exactly 60 feet away from the beholdragon, some creatures will be closer to it when they get restrained by the tentacles; and I imagine these tentacles being strong enough to hold most restrained creatures in a way that they aren't just dangling below the beholdragon.

Swallow Whole is cool, though there's no limit to the number of creatures it can swallow, was this intentional?

Swallow Whole targets only one creature, so it can only swallow one creature each time it uses Swallow Whole.

Squeeze seems like it would fit better in the Legendary actions, giving players a chance to try and get away first, purely from a gameplay perspective. mechanically it's sound!

Changed Squeeze to be a loegendary action, while Gaze is now a bonus action, reading

Gaze. The beholdragon stares at a creature that became invisible or used the Hide action during the last round. That creature must make a DC 23 Wisdom saving throw or take 18 (4d8) psychic damage and be frightened until the end of its next turn as it realizes that it can't escape the beholdragon's dreadful gaze.

 instead of a legendary action (which also makes more sense since hiding from the beholdragon basically was impossible anyways with it having eyes everywhere).

All in all, I like it a lot, and will likely try it out in a high level campaign! I am already formulating a plot aorund it, involving a friendly beholder who the kingdom needs saving from their nightmares, but will the players be too late?

I hope your players will like it... and I hope they can stomach all the sanity damage they will take...
I am also planning a quest around it to run this monster in my local AL group - we are less strict than official AL, so homebrew adventures are fine - which I also ran Don't Say Vecna for with several homebrew changes including a 2-phase boss fight using my Vecna ("Archlich" here on Beyond) statblock.

In my case the beholdragon likely would have its lair in the ruins of an old fortress not too far away from a small, idyllic farming village in mountainous terrain, where villagers are slowly succumbing to madness as strange things happen and their apple trees suddenly grow eyeballs instetad of apples  - which actually is an amazing pun in my native language, German, because we call an eyeball an "Augapfel" or "eye-apple".

I will publish the updated statblock soon, just waiting if you want to answer my questions so that I can implement these changes :-)

Also, now that I am thinking of it, your Shadowstalker could be an amazing minion for the beholdragon. It always sees you, you always feel being watched... but you don't know what it is...

Quote from Semako >>

Quote from ThorukDuckSlayer >>

Neverending nightmares is a cool concept, I might have gone for an increasing DC (starting at 12 and increasing by 2 for each long rest), with a success allowing you to rest a bit (half health regained, and half spell slots restored), to give it a more creeping-up effect.

First, thank you a lot for your feedback!! I am glad you like this statblock, and I hope you will have some sweet dreams tonight... :-D

I will change that, thank you for the suggestion. I will start the DC at 10 and increase it by 5 for each attempted rest, which each fail resulting in a long-term madness. After all, a character who rests for multiple days in such an area deserves to get a multitude of madnesses...

Cone of Draconic Wrath is very nice, and I like how it allows for a range of beholdragons. The mention of secondary effects makes me wonder if this needs to clarify that it's the primary breath weapon (EG not the repulsion breath or enervating breath).

Yes, it indeed means the primary breath weapon. With the secondary effects I was thinking of gem dragons and other non-metallic/non-chromatic ones that have breath weapons with secondary effects (also, I will soon have Sardior's statblock finished!). I am not sure though how I could clarify that in an easily understandable way. On the other hand, you could, if you want, combine the effects of a metallic's primary and secondary breath weapon into the Cone of Dragon's Wrath effect.

Eyestalks, I love that you can cut the eyestalks off. That's a good way to make a boss fight more interesting!

Many eyes menace is very creepy, love it.

I am glad you like those effects!

Nightmare ray is going to have players scrambling to get to one another to wake each other up, which is an awesome thing to mix into a fight!

Pain ray is a good way to shut down casters.

I like the Maiming ray, but I would consider having the tentacle be functional, and permanent. Otherwise clever play risks being punished rather than rewarded! "yes, but..." rather than "Actually, no."

Draconic Ray is another nod to the original dragon, helps keep it thematic.

Changed the effect of maiming ray to produce an actually "somewhat functional" tentacle. I am glad you like the other eye rays!

Eyeful presence - Typo (Whie frightened in this way...),  otherwise I like it, especially it invoking Confusion. It is a little concerning that it could literally wipe a party with this one move, as it can do it every turn. Scary is one thing, but perhaps have whoever rolls the lowest be affected by Confusion, rather than everyone? This one ability can drop the party to only having a 20% chance of actually playing this round, and I have to say, that doesn't sound fun for the players!

Typo is fixed already, thanks for pointing it out. Right now it has the confusion effect for everyone who fails and those who fail by 5 instead gain a short-term madness. Not sure how to change it. On one hand, if the party works together, fear can easily be avoided and cured - such as by a heroes' feast spell, a paladin's aura, magic items and other things. On the other hand, you are right, that if the party has bad dice luck, the situation, well, gets dicey fast. In general when I write monster statblocks I always expect that DMs adjust the monster's DC to their party to avoid impossible saving throws, so there should not be a situation where a party is not able to get out of this kind of fear.       
Maybe the way to go is to remove the madness and have the confusion proc when a creature fails by 5 or more? Or remove the confusion and have just the madness effect on a fail by 5 or more? I am also considering lowering a creature's Wisdom stat (similar to Hastur's Reveal Visage, although without paralysis) as this thing is so otherworldly and defies their understanding of the world?

Reality Shattering Landing is an awesome image, I feel it needs split rules for whether you're landed on or next to, I feel like landing on someone should do more damage. There's also need for something to say "after this move, move any creatures which share a space with the beholdragon the minimum distance needed to no longer share a space."

While you are right, I think I am keeping it like that, as it is simpler, more streamlined. It also is how the similar ability for my great wyrms, Bahamut and Tiamat works. Basically, the area around the dragon is what I imagine is covered by the dragon's wings (therefore it is the same radius as the dragon's wing attack legendary action if it has one). I could add that creatures are pushed away, but interpreting it as the creatures are still within the dragon's space and have to move out of it could work too, especailly for bigger dragons where creatures could feasibly be prone beneath the dragon's body.

Antimagic Breath, I wonder if it could have lingering effects on casters - they need to pass a DC23 check with their spellcasting modifier when the ycast a spell or the spell is wasted, and the first time they succeed the effect ends? something like that could have them popping cantrips in a desperate attempt to break out of the antimagic!

Interesting idea, but I think this would be a bit too harsh? It basically shuts them down completely until they make the check or saving throw (I would go for a saving throw here so that they can actually be proficient and beneit from other saving throw bonuses like a paladin's aura, a bless spell and other abilities). Having all spells of 5th level or lower breathed away already is enough in my opinon. It means spells like freedom of movement, [Tooltip Not Found], haste, wall of force, fly, protection from evil and good and many more that would be quite powerful against the beholdragon are gone now, the time used to set them up basically is wasted. Also, the pain ray already cripples casters hard *evil DM laughter*

Tentacle breath, I like the way it works, I feel it could use a little more clarification on restrained creatures moving with the beholdragon - perhaps "after the beholdragon moves, any creatures currently restrained by the tentacle breath are moved towards the dragon until they are 60ft. away. If the beholdragon is flying, they hang 60ft. below it."

I already clarified it a bit more, although I left the part about characters hanging below the dragon out. Not every creature will get caught exactly 60 feet away from the beholdragon, some creatures will be closer to it when they get restrained by the tentacles; and I imagine these tentacles being strong enough to hold most restrained creatures in a way that they aren't just dangling below the beholdragon.

Swallow Whole is cool, though there's no limit to the number of creatures it can swallow, was this intentional?

Swallow Whole targets only one creature, so it can only swallow one creature each time it uses Swallow Whole.

Squeeze seems like it would fit better in the Legendary actions, giving players a chance to try and get away first, purely from a gameplay perspective. mechanically it's sound!

Changed Squeeze to be a loegendary action, while Gaze is now a bonus action, reading

Gaze. The beholdragon stares at a creature that became invisible or used the Hide action during the last round. That creature must make a DC 23 Wisdom saving throw or take 18 (4d8) psychic damage and be frightened until the end of its next turn as it realizes that it can't escape the beholdragon's dreadful gaze.

 instead of a legendary action (which also makes more sense since hiding from the beholdragon basically was impossible anyways with it having eyes everywhere).

All in all, I like it a lot, and will likely try it out in a high level campaign! I am already formulating a plot aorund it, involving a friendly beholder who the kingdom needs saving from their nightmares, but will the players be too late?

I hope your players will like it... and I hope they can stomach all the sanity damage they will take...
I am also planning a quest around it to run this monster in my local AL group - we are less strict than official AL, so homebrew adventures are fine - which I also ran Don't Say Vecna for with several homebrew changes including a 2-phase boss fight using my Vecna ("Archlich" here on Beyond) statblock.

In my case the beholdragon likely would have its lair in the ruins of an old fortress not too far away from a small, idyllic farming village in mountainous terrain, where villagers are slowly succumbing to madness as strange things happen and their apple trees suddenly grow eyeballs instetad of apples  - which actually is an amazing pun in my native language, German, because we call an eyeball an "Augapfel" or "eye-apple".

I will publish the updated statblock soon, just waiting if you want to answer my questions so that I can implement these changes :-)

Also, now that I am thinking of it, your Shadowstalker could be an amazing minion for the beholdragon. It always sees you, you always feel being watched... but you don't know what it is...