Cone of Draconic Wrath. The beholdragon's central eye emits a magical, 150-foot cone of terror and draconic energy of the same elemental power as the dragon the beholder was dreaming of when the beholdragon came into existence. At the start of each of its turns, the beholdragon decides which way the cone faces and whether the cone is active.
Each creature that starts its turn within the area must succeed on a DC 23 Constitution saving throw,  taking 27 (6d8) damage of that dragon's breath weapon damage type on a failure or half as much on a success. In addition, on a  failure, the target suffers any secondary effects that dragon's breath weapon may have had.

Empowered Attacks. The beholdragon's weapon attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks.

Eyestalks. When the beholdragon takes a critical hit, it must succeed on a Constitution saving throw with a DC equal to the damage taken or loses an eyestalk, and with it, it loses the use of a random eye ray. After one minute, or when the beholdragon uses its Horrifying Regeneration reaction, the severed eyestalk regrows.

Legendary Resistance (3/Day). If the beholdragon fails a saving throw, it can choose to succeed instead.

Magic Resistence. The beholdragon has advantage on saving throws against spells and magical effects.

Many-Eyed Menace. The beholdragon can see through the eyes of any creature within 5 miles of it, unless such a creature is protected by a mind blank spell or similar magic. In addition, as an action, the beholdragon can open an eye on any solid surface within 5 miles of its lair and see through it as if it was its own eye.

Treasure Sense. The beholdragon knows the location of all magic items and of accumulations of mundane treasure within 5 miles of it.

Multiattack. The beholdragon can use its Eyeful Presence, and if it was flying at the start of its turn, it can use its Reality-Shattering Landing. It then makes two attacks with its claws and uses its eye rays. If it has creatures restrained by its tentacle breath, it can use its Swallow Whole in place of one claw attack.

Claw. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 28 (2d10 + 7) slashing damage. If the target is a creature, it must succeed on a DC 23 Strength saving throw or be knocked prone.

Tentacle Tail. The dragon swings its tentacly tail in a 30-feet cone behind it. Each creature within the area must succeed on a DC 23 Dexterity saving throw or take  16 (2d8+7) bludgeoning damage plus 18 (4d8) psychic damage and is knocked prone.

Eye Rays. The beholdragon shoots three of the following magical eye rays at random (reroll duplicates), choosing one to three targets it can see within 120 feet of it:

• 1. Dominating Ray.The targeted creature must succeed on a DC 23 Wisdom saving throw or must use its reaction to move up to its speed and make a weapon attack against a creature of the beholdragon's choice. A creature immune to the charmed is immune to this effect.
  
• 2. Slowing Ray. The targeted creature must make a DC 23 Dexterity saving throw. On a failed save, the target's speed is halved for 1 minute. In addition, the creature can't take reactions, and it can take either an action or a bonus action on its turn but not both. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
  
• 3. Nightmare Ray. The targeted creature must succeed on a DC 23 Wisdom saving throw or fall  asleep and remain unconscious for 1 minute. While it remains unconscious, it suffers horrifying nightmares and takes 27 (6d8) psychic damage at the start of each of its turns. The target awakens if it takes damage from any other source or if another creature takes an action to wake it. This ray has no effect on constructs and undead.
• 4. Mindfire Ray. The targeted creature must succeed on a DC 23 Intelligence saving throw or take 35 (10d6) psychic damage and has to subtract 1d6 from its attack rolls and saving throws for 1 minute. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
• 5. Pain Ray. If the target is a creature, it must succeed on a DC 23 Constitution saving throw or be affected by crippling pain. While the target is affected by crippling pain, any speed it has can be no higher than 10 feet. The target also has disadvantage on attack rolls, ability checks, and saving throws, other than Constitution saving throws. Finally, if the target tries to cast a spell, it must first succeed on a DC 23 Constitution saving throw, or the casting fails and the spell is wasted.
  A target suffering this pain can repeat the saving throw at the end of each of its turns. On a successful save, the pain ends.
• 6. Maiming Ray. The targeted creature must succeed on a DC 23 Constitution saving throw or take 36 (8d8) necrotic damage and be unable to regain hit points until the start of the Beholdragon's next turn. Furthermore, if the creature fails the saving throw by 5 or more, one of its limbs is dissolved into inky black ooze, with the effect of such loss determined by the DM. If a creature attempts to use magic to restore the lost limb within 1 hour of its loss, the creature must succeed on a DC 23 Wisdom saving throw or a somewhat functional tentacle grows in place of the lost limb. If this happens, the original limb can only be restored with a wish spell or similar magic.
• 7. Telekinetic Ray. If the target is a creature, it must succeed on a DC 23 Strength saving throw or be flung up to 60 feet in a straight direction. If it hits a solid surface such as a wall, floor or ceiling, it takes 1d6 bludgeoning damage and 1d6 psychic damage for every 10 feet it was thrown before impact and lands prone. If the target would collide with another creature instead, that other creature must succeed on a DC 19 Dexterity saving throw or take the same damage and be knocked prone too.
• 8. Draconic Ray. This eye ray bears some of the same elemental power as the dragon the beholder was dreaming of when the beholdragon came into existence. The targeted creature must succeed on a DC 23 Dexterity saving throw or take 55 (10d10) damage of that dragon's breath weapon damage type.
  

Eyeful Presence. The beholdragon opens its maw and the giant eyeball within it dreadfully gazes at its enemies as do all the beholdragon's eyestalks. Each creature of the dragon's choice that is within 120 feet of the dragon and can see it must succeed on a DC 23 Wisdom saving throw or be frightened. In addition, if a creature fails the saving throw by 5 or more, it gains a short-term madness. It can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the beholdragon's Eyeful Presence for the next 24 hours.

Reality-Shattering Landing. If the beholdragon is flying, it can attempt to land forcefully in a space of its choice, even if that space is occupied by other creatures. When it does so, its movement speed is reduced to 0 until the end of the turn, and shockwaves of aberrant magic ripple through the ground around it, causing it to feel soft, slimy or otherwise surreal, sometimes even illusions of the former dragon's hoard flicker through the area. The ground within 20 feet of the beholdragon's space becomes difficult terrain until the start of the beholdragon's next turn and any creature in the beholdragon's space or within 20 feet of it must succeed on a DC 23 Dexterity saving throw or takes 18 (4d8) psychic damage and 18 (4d8) bludgeoning damage and is knocked prone.

Breath Weapon (Recharge 5–6). The beholdragon uses one of the following breath weapons; it cannot use the same breath weapon on two consecutive turns:

• Antimagic Breath: The beholdragon opes its maw and the giant eyeball within emits a wave of magical energy in a 90-feet cone. Each creature within the area must make a DC 23 Constitution saving throw, taking 78 (12d12) force damage on a failure or half as much on a success. Every spell of 5th level or lower within the area ends.
• Tentacle Breath. Tentacles shoot forward from the beholdragon's maw within a 60-feet cone. Each creature within the area must make a DC 23 Strength savong throw. On a failure, it takes 36 (8d8) bludgeoning damage and 36 (8d8) damage of the breath weapon damage type of the dragon the beholder was dreaming of and is restrained by the tentacles. A restrained creature can use its action to attempt to escape with a successful DC 17 Strength (Athletics) check.
  The up to 60 feet long tentacles remain attached to the beholdragon until it retracts them (no action required), restrained creatures move with the beholdragon, held by the tentacles.

Swallow Whole. Melee Weapon Attack: +15 to hit, reach 60 ft., one target restrained by the beholdragon's tentacle breath. Hit: 20 (2d12 + 7) piercing damage, and if the creature is huge or smaller, the beholdragon pulls it into its maw and swallows it.

While swallowed, the creature is blinded and restrained and has total cover against attacks and other effects outside the beholdragon, except for eye rays - the beholdragon can still target a creature it has swallowed with them. In addition, it takes 27 (6d8) acid damage and 27 (6d8) psychic damage at the start of each of the beholdragon's turns.

If the beholdragon takes 50 damage or more on a single turn from a creature inside it, the beholdragon must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the beholdragon. If the beholdragon dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

As soon as a swallowed creature escapes from the beholdragon, regardless of whether it was regurgitated or escapes in some other way, that creature must succeed on a DC 23 Wisdom saving throw or suffers a short-term madness.

Gaze. The beholdragon stares at all creatures that became invisible or took the Hide action during the last round. Each target must succeed on 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.

Horrifying Regeneration. When the beholdragon takes damage of the breath weapon damage type of the dragon the beholder was dreaming of, it can use its reaction to gain immunity to that instance of damage. If it does so, it regains an amount of hit points equal to the damage it would have taken and instantly regenerates one severed eyestalk.

The beholdragon can take 5 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The beholdragon regains spent legendary actions at the start of his turn.

Eye Ray. The beholdragon shoots one random eye ray.

Squeeze. The beholdragon physically and mentally squeezes and crushes creatures restrained by it. Each creature restrained by it must make a DC 23 Constitution saving throw or take 14 (3d8) psychic damage and 14 (3d8) bludgeoning damage and start suffocating.

Devour (Costs 2 Actions). The beholdragon uses its Swallow Whole.

Dreadful Shriek (Costs 2 Actions). The beholdragon lets out a dreadful, otherworldly shriek, filling its enemies' heads with pain and terror. Each creature within 120 feet of the Beholdragon must succeed on a DC 23 Wisdom saving throw or take 18 (4d8) psychic damage and must immediately use its reaction to move as far away from the beholdragon as its movement speed allows. The creature does not move into obviously dangerous terrain, such as fire or a pit.

Tail Swing (Costs 2 Actions). The beholdragon attacks with its tentacle tail.

Collapse Distance (Costs 3 Actions). The beholdragon unleashes a surge of aberrant magic, teleporting itself to an unoccupied space within 60 feet of it. Each creature within 20 feet of the space the beholdragon left must succed on a DC 23 Strength saving throw or takes 27 (6d8) force damage and is knocked prone.