Beholder Dragon

Anti-magic Cone
The beholder dragon's central eye creates an area of anti-magic, as in the anti-magic spell, in a 150-ft cone. At the start of each of its turn, the beholder dragon decides which way the cone is facing, and whether it is active or not. The area works against its own eye beams, and targets inside of it are not affected by banishing breath.

Multi-attack
The beholder dragon may use its Frightful Presence if it chooses and then makes three attacks; one with its bite and two with its claws.

Frightful Presence
Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 16 Wisdom saving throw or become Frightened for 1 minute. A creature 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 dragon's Frightful Presence for the next 24 hours.

Bite Melee Weapon Attack: +11 to hit, 10-ft range, one target. Hit: 4d6+8 piercing damage

Claw Melee Weapon Attack: +11 to hit, 5-ft range, one target. Hit: 2d6+8 bludgeoning damage

Banashing Breath
The beholder dragon exhales a dimensional rift in a 90-ft straight line that is 10-ft wide. Each creature in that line must make a DC 18 Charisma Saving Throw, or be banished. If the beholder dragon uses its breath attack again before the duration of the effect ends, all other previously affected targets re-appear. 

Eye Rays
See Legendary Actions

Tail Swipe Melee Weapon Reaction Attack: +11 to hit, 30-ft radius around. Hit: 3d6+8 bludgeoning damage

Legendary Resistance (3/day)
If the beholder dragon fails on a saving throw, attack, or ability check, it may choose to automatically succeed. The beholder dragon may also choose this ability to negate any damage towards it.

Eye Rays (cont.)
The beholder shoots three of the following magical eye rays at random (re-roll duplicates), choosing one to three targets it can see within 120-ft of it:

Charm Ray: The targeted creature must make a DC 16 Wisdom Saving Throw or be charmed by the beholder for 1 hour, or until the beholder harms the creature

Paralyzing Ray: The targeted creature must make a DC 16 Constitution Saving Throw or be paralyzed for 1 minute. The target can repeat the saving throw on the end of its turn, ending the effect on itself upon successful save.

Fear Ray: The targeted creature must make a DC 16 Wisdom Saving Throw or be frightened  for 1 minute. The target can repeat the saving throw on the end of its turn, ending the effect on itself upon successful save.

Slowing Ray: The targeted creature must make a DC 16 Dexterity Saving Throw. On a failed save, the target's speed is halved for 1 minute. In addition, the creature can't make any reactions, and can only make an attack or bonus action on its turn, not both. The creature can repeat the saving throw on the end of its turn, ending the effect on itself upon successful save.

Enervation Ray: The targeted creature must make a DC 16 Constitution Saving Throw, taking 8d8 Necrotic Damage on a failed save, or half as much on a successful save.

Telekinetic Ray: If the target is a creature, it must succeed on a DC 16 Strength Saving Throw or the beholder moves it up to 30-ft  in any direction. It is restrained by the ray's telekinetic grip until the start of the beholder's next turn, or if the beholder is incapacitated. If the target is an object weighing 300 pounds or less that isn't being worn or carried, it is moved up to 30-ft in any direction. The beholder can exert fine control on objects using this ray, such as manipulating a simple tool or opening a door.
 
Sleep Ray: The targeted creature must succeed on a DC 16 Wisdom Saving Throw or fall asleep and remain unconscious for 1 minute. The target wakens if it takes damage or another creature takes an action to wake it. This ray has no effect on constructs and undead.

Petrification Ray: The targeted creature must make a DC 16 Dexterity Saving Throw. On a failed save, the creature begins to turn to stone and 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 until freed by the greater restoration spell, or other magic.

Disintegration Ray: If the target is a creature, it must succeed on a DC 16 Dexterity Saving Throw or take 10d8 force damage. If the this damage reduces the creature to 0 hit points, its body becomes a pile of fine gray dust. If the target is a large or smaller, non-magical  object or creation, it is disintegrated without a saving throw. If the target is a huge or larger target, it disintegrates a 10-ft cube of that object.

Death Ray: The targeted creature must succeed on a DC 16 Dexterity Saving Throw or take 10d10 Necrotic damage. The target dies if it reduces the target's hp to 0.