Astral Dreadnought Beholder

Astral Entity. The astral dreadnought beholder can't leave the Astral Plane, nor can it be banished or otherwise transported out of the Astral Plane.

Demiplanar Donjon. Any creature or object that the astral dreadnought swallows is transported to a demiplane that can be entered by no other means except a wish spell or this creature's Donjon Visit ability. A creature can leave the demiplane only by using magic that enables planar travel, such as the plane shift spell. The demiplane resembles a stone cave roughly 1,000 feet in diameter with a ceiling 100 feet high. Like a stomach, it contains the remains of the dreadnought's past meals.

The dreadnought beholder can't be harmed from within the demiplane. If the dreadnought dies, the demiplane disappears, and everything inside it appears around the corpse. The demiplane is otherwise indestructible.

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

Magic Weapons. An astral dreadnought's weapon attacks are magical.

Sever Silver Cord. If the astral dreadnought beholder scores a critical hit with its claw attack against a creature traveling through the Astral Plane by means of the astral projection spell, the dreadnought can cut the target's silver cord instead of dealing damage.

Multiattack. The astral dreadnought makes three attacks: one with its bite and two with its claws. 

Bite. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 36 (5d10 + 9) piercing damage. If the target is a creature of Huge size or smaller and this damage reduces it to 0 hit points or it is incapacitated, the astral dreadnought swallows it. The swallowed target, along with everything it is wearing and carrying, appears in an unoccupied space on the floor of the astral dreadnought's Demiplanar Donjon.

Claw. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 19 (3d6 + 9) slashing damage.

Eye Rays The dreadnought beholder shoots three of the following magical eye rays at random (reroll duplicates), choosing one to three targets it can see within 120 feet. Choose ray 1d10
1. Charm Ray The targeted creature must succeed on a DC 16 Wis save or be charmed by the Dreadnought beholder for 1 hour, or until the beholder harms the creature
2. Paralyzing Ray The targeted creature must succeed on a DC 16 Con save or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on a success.
3. Fear Ray The targeted creature must succeed on a DC 16 Wis save or be frightened for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on a success.
4. Slowing Ray The targeted creature must succeed on a DC 16 Dex save. 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 it's turn, not both. The target can repeat the saving throw at the end of each of its turns, ending the effect on a success.
5. Enervation Ray The targeted creature must make a DC 16 Con save, taking 36 8d8 necrotic damage on a failed save, or half as much on a successful one.
6. Telekinetic Ray If the creature is a creature, it must succeed on a DC 16 Str save or the dreadnought beholder moves it up to 30ft in any direction. It is restrained by the ray's telekinetic grip until the start of the beholder's next turn or until 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 30ft in any direction. The beholder can also exert fine control on objects with this ray, such as manipulating a simple tool or opening a door or a container.
7. Sleep Ray The targeted creature must succeed on a DC 16 Wis save or fall asleep and remain unconscious for 1 minute. The target awakens if it takes damage or another creature takes an action to wake it. This ray has no effect on constructs and undead.
8. Petrification Ray The targeted creature must make a DC 16 Dex save. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of it's 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.
9. Disintegration Ray If the target is a creature, it must succeed on a DC 16 Dex save or take 45 (10d8) 10d8 force damage. If this creature is reduced to 0 hit points, it's body becomes a pile of fine gray dust. If the target is a large or smaller non-magicall object or creation of magical force, it is disintegrated without a savein throw. If the target is Huge or larger or creation of magical force, this ray disintegrates a 10ft cube of it.
10. Death Ray The targeted creature must succeed on a DC 16 Dex save or take 55 (10d10) 10d10 necrotic damage. The target dies if the ray reduces it to 0hit points.