Ascendant Nightmare Beholder

Legendary Resistance (3/Day, or 4/Day in Lair). If the beholder fails a saving throw, it can choose to succeed instead.

World Bleed. The area within 300 feet of the Beholder is warped. Natural laws are bent: Magic is unreliable. Every spell cast within this range requires a DC 18 ability check using the caster’s spellcasting ability. On a failure, the spell fizzles and the caster takes 4d10 psychic damage.
Nonmagical healing has no effect.
Saving throws against Frightened effects are made with disadvantage. Creatures immune to the frightened condition make saving throws against it with advantage.

Inner Reality. The beholder is immune to any spell or effect that would alter its form or forcibly teleport/change its position against its will. 

Horrible Revelation. When a creature first sees the Beholder each day, it must succeed on a DC 23 Wisdom saving throw or immediately suffer one random Long-Term Madness (DMG pg. 260) and take 40 (8d10) psychic damage.

Blood of the Dreaming Abyss. When damaged by piercing or slashing attacks, the Beholder erupts with living blood. Creatures within 30 feet must succeed on a DC 23 Dexterity saving throw or take 30 (6d10) necrotic damage and become dream-cursed for 1 minute.
While dream-cursed they have disadvantage on Intelligence, Wisdom, and Charisma checks and saving throws. And their dreams are controlled by the Beholder if they fall [condition]unconscious[/unconscious] as though through the dream spell.

Dread Consumption.
Each time the Beholder deals psychic or necrotic damage to a creature, it gains 1 Dread Stack. When the Beholder reaches 5 stacks it can choose to immediately:

• Recharge all expended Eye Rays, and regain all expended legendary actions if it is not its turn.
• Fire two different Eye Rays as a bonus action on it's current turn. (Or upcoming turn)
• All creatures that can see the Beholder must succeed a DC 21 Wisdom saving throw or become Frightened for 1 minute. Remaking the saving throw at the end of each of their turns.
• After activation, all stacks are reset to 0.

Bite. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 49 (12d6+7) piercing damage.

Nightmare Eye Rays. The beholder shoots five of the following magical eye rays at random (reroll duplicates), choosing one to five targets it can see within 150 feet of it:

1. Twilight Charm Ray: The targeted creature must succeed on a DC 24 Wisdom saving throw or be charmed and suffer hallucinations for 1 minute. While charmed, the creature perceives its allies as monstrous enemies and its enemies as loved ones begging for help. The creature must repeat the saving throw at the end of each of its turns, ending the effect on a success.

2. Paralyzing Grief Ray: The target must succeed on a DC 24 Constitution saving throw or be paralyzed for 1 minute as spectral hands of dead loved ones grasp at them. The creature can repeat the saving throw at the end of each of its turns, with disadvantage if it has the frightened or charmed conditions.

3. Dread Infusion Ray: The creature must make a DC 24 Wisdom saving throw or become frightened and take 28 (5d8) psychic damage, or half as much on a success. While frightened in this way, the target cannot distinguish illusion from reality and has disadvantage on all Intelligence, Wisdom, and Charisma checks and saving throws for the duration (1 minute). Repeating the save at end of each of its turns.

4. Sloth Ray: The targeted creature must succeed on a DC 24 Dexterity saving throw or be cursed with unbearable mental exhaustion. For 1 minute, the creature’s speed is halved, it cannot take reactions, and can only take an action or bonus action on its turn—not both. A creature can repeat the saving throw at the end of each of their turns. Regardless of success or failure, the creature takes 31 (5d8) necrotic damage.

5. Soulleech Ray: The target must succeed on a DC 24 Constitution saving throw or take 23 (5d8) poison damage and 23 (5d8) necrotic damage and lose 1 hit die. If the creature has no hit dice remaining, it gains 1 level of exhaustion instead. The beholder then regains hit points equal to a roll of its own hit dice (1d20+10).

6. Mental Shackles Ray: The targeted creature must succeed on a DC 24 Strength saving throw or become Restrained as phantom chains of regret and sorrow wrap around them. While restrained, the creature has disadvantage on saving throws against other eye rays. The condition lasts until the end of the Beholder’s next turn or until Dispel Magic is used targeting them.

7. Sleep Paralysis Ray: The creature must succeed on a DC 24 Wisdom saving throw or fall into a waking nightmare, becoming unconscious for 1 minute. The creature remains aware of its surroundings but cannot act. It awakens if it takes damage from an external source or another creature uses an action to wake it. While in this state the creature is afflicted by nightmarish hallucinations behind their woken eyes taking 10 (3d6) psychic damage at the beginning of each of their turns. This damage does not awaken the creature.

8. Shatterform Ray: The creature must make a DC 24 Dexterity saving throw or begin turning to black crystal. On a failure, it is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified in an obsidian-like stasis until freed by greater restoration spell or other magic such as wish. While petrified in this way the beholder has advantage on Bite attacks against the creature and deals double damage with said attacks.

9. Oblivion Ray:  The creature must succeed on a DC 24 Dexterity saving throw or take 66  (12d10) force damage. If this reduces the creature to 0 HP, they are obliterated into living shadow, unable to be resurrected except by the wish or true resurrection spell. Magical constructs, undead or summoned creatures are instantly destroyed.

   If the target is a Large or smaller nonmagical object or creation of magical force, it is disintegrated without a saving throw. If the target is a Huge or larger object or creation of magical force, this ray disintegrates a 15-foot cube of it.

10. Heartstopper Ray: The creature must succeed on a DC 24 Constitution saving throw or take 88 (16d10) necrotic damage. If reduced to 0 HP, the creature dies instantly, and its heart is transmuted into a twitching orb of darkness that explodes when a creature starts its turn within 5 feet, or comes within 5 feet of orb; dealing 3 (1d6) necrotic damage to all creatures within 5 feet.
    A creature killed this way cannot be resurrected unless their heart is healed using the regenerate spell. Or if the wish or true resurrection spells are used.

11. Soul Rot Ray: The creature must succeed on a DC 24 Charisma saving throw or it's hit point maximum is halved for 24 hours or until the greater restoration or similar magic is used. The hit points lost, if any, when the creatures maximum is halved do not heal from the restoration unless its done via wish. 

12. Unmaking Ray: The creature must succeed on a DC 24 Dexterity saving throw or take 100 (20d10) force damage. If this damage reduces the creature to 0 hp, they are erased from existence (no death saves). The only spell capable of bringing them back is the wish spell.

Quell Reality. When a creature within 300 ft. of the beholder rolls a 20 on an attack roll, ability check, or saving throw; the beholder can choose to turn the roll into a 1.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature's turn , the beholder can expend a use to take one of the following actions. The beholder regains all expended uses at the start of each its turns.

Eye Ray. The beholder uses one random eye ray.

Nightmare Ray (Costs 2 Actions): The Beholder shoots its large Eye Ray at a target within 150 feet. The target must make a DC 21 Constitution saving throw or take 8d10 psychic damage and suffer Disadvantage on all Wisdom, Intelligence, and Charisma saving throws until the end of the Beholder's next turn.

Reality Warp (2 Actions). Twist space; each creature within 60 feet must succeed on a DC 21 Intelligence saving throw or be teleported 1d100 (rounded down to the nearest increment of 5) feet in a direction of the beholders choice, taking 6d10 force damage.

Flood the Dream (3 Actions). All cursed creatures (dream-cursed) automatically take 40 psychic damage and are stunned until the end of their next turn.

Mythic Trait: Dream Rebirth
If reduced to 0 hit points, the Nightmare Beholder doesn't die immediately. Instead it sinks into its own blood, vanishing into the floor.

3 rounds later, it erupts back into reality at full health, but now reality itself has become part of the Dream of Nightmares within a 1-mile radius permanently.

Players must now find a way to seal the dream or escape, lest they be trapped inside the nightmare world forever as the Beholder becomes immune to damage until the dream is sealed. (If the dream is sealed the Beholder regains 445 hit points and becomes killable)

Award a party an additional 100,000 XP for defeating the beholder after its Dream Rebirth activates.

Despair Surge — "Dream of the Unseen Grave"
When the Beholder reaches 5 stacks of Dread Consumption and maintains them for 2 consecutive rounds without expending them, it triggers a Despair Surge — an even more terrifying transformation.

Roll 1d4 to determine the effects:

1. Reverse Gravity Collapse
   The Beholder distorts the very space around it, pulling all creatures within 60 ft. (except itself) into a chaotic, upside-down world. Each creature must make a DC 21 Strength saving throw or be pulled into the air (up to 60 ft.) and take 6d10 force damage. On a success, they take half damage and fall prone.
   For 1 minute, creatures that fail this save are incapacitated while floating in the distorted gravity field.
   
   
2. Memory Leak
   The Beholder unleashes an aura of psychic devastation that rends the minds of its enemies. Each creature within 60 ft. must make a DC 20 Intelligence saving throw or suffer the loss of 1 ability score (player’s choice) for 1 hour. The effect is cumulative, and the creature must make the save at the start of each of its turns until it succeeds. On a success, the creature suffers no penalty but is Frightened for the duration.
   
   
3. Shadow Bloom
   Terrifying, thorned tendrils made of darkness burst forth in a 60 ft. radius around the Beholder. Each creature in the area must succeed a DC 22 Dexterity saving throw or take 8d10 piercing + 4d10 necrotic damage. Those who fail are also grappled and must spend an action to escape the tendrils (DC 20 Strength check).
   
   
4. Echo of the Forgotten
   The Beholder summons a twisted, corrupted reflection of one of its foes (chosen by the DM or based on a fallen PC). This shadowy reflection is hostile and uses the same stats as the creature’s original form but has the Fiendish subtype and is under the Beholder's control. The shadow acts on initiative count 20 (losing initiative ties) and attacks for 1 minute before crumbling into nothingness.