Antiweapon Cone. Xanathar's central eye creates a 150 foot cone. All weapon attacks in that area have disadvantage. This also creates the effect of an Antimagic Field spell. At the start of each of its turns, Xanathar decides which way the cone faces and whether the cone is active. The area works against his own eye rays as well.

Items. Xanathar is adorned by golden items he has collected through the ages. Some of these are merely accessories, but most are magic rings for his protection attached to its eye stalks.

• Ring of Protection:  +1 to AC & +1 to all saving throws (both included in stats already)
• Ring of Evasion:  3 charges regaining 1d3 daily at dawn. Can expend 1 charge and use a reaction on a failed Dexterity save to succeed instead
• Ring of Fire Resistance:  Gains resistance to fire damage
• Ring of Attunement:  Grants one additional attunement slot (requires attunement, itself)

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 14 (5d6+4) piercing damage.

Spellcasting. Xanathar is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 22,+14 to hit with spell attacks). Xanathar has the following wizard spells from Xanathar's Guide to Everything prepared:

Cantrips (at will): create bonfire, frostbite, shape water

1st level (4 slots): absorb elements, cause fear, earth tremor

2nd level (3 slots): Aganazzar's scorcher, dragon's breath, dust devil, shadow blade

3rd level (3 slots): catnap, erupting earth, flame arrows, wall of water

4th level (3 slots): elemental bane, sickening radiance, summon greater demon, vitriolic sphere

5th level (3 slots): control winds, dawn, infernal calling

6th level (2 slots): create homunculus, mental prison

7th level (1 slot): power word pain, whirlwind

8th level (1 slot):  Abi-Dalzim's horrid wilting, illusory dragon

9th level (1 slot): invulnerability, mass polymorph

Eye Rays. The beholder 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. Charm Ray. The targeted creature must succeed on a DC 22 Wisdom saving throw or be charmed by the beholder for 1 hour, or until the beholder harms the creature.
2. Paralyzing Ray. The targeted creature must succeed on a DC 22 Constitution saving throw 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 itself on a success.
3. Fear Ray. The targeted creature must succeed on a DC 22 Wisdom saving throw 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 itself on a success.
4. Slowing Ray. The targeted creature must succeed on a DC 22 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, 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.
5. Enervation Ray. The targeted creature must make a DC 22 Constitution saving throw, taking 44 (8d10) necrotic damage on a failed save, or half as much damage on a successful one.
6. Telekinetic Ray. If the target is a creature, it must succeed on a DC 22 Strength saving throw or the beholder moves it up to 30 feet 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 30 feet 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 22 Wisdom saving throw 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 22 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.
9. Disintegration Ray. If the target is a creature, it must succeed on a DC 22 Dexterity saving throw or take 55 (10d10) force damage. If 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 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 10-foot cube of it.
10. Death Ray. The targeted creature must succeed on a DC 22 Dexterity saving throw or take 65 (10d12) necrotic damage. The target dies if the ray reduces it to 0 hit points.

At the end of Xanathar's turn, it takes an extra bonus action to use up to 3 legendary actions.

Ring of Evasion.  When you fail a Dexterity saving throw while wearing it, you can use your Reaction to expend 1 of its 3 Charges to succeed on that saving throw instead.

Ouroboros Ring. When a creature that you can see (including yourself) rolls a d20, you can use your reaction to speak the ring's command word and roll a d8. You make this decision after knowing the result of the roll, but before the GM declares the outcome.

When you do, you alter the number rolled on the d20, before any modifiers are added to it. You can choose to either add or subtract the number you rolled on the d8 to or from the d20. The d20 is treated as if it had originally rolled the altered number. If the d8 would cause the number rolled on the d20 to be greater than 20 or less than 1, continue to count up or down as though the next number after 20 were 1, or the number below 1 were 20. For example, a creature rolls a 19 on its d20 for an attack roll against you, before any modifiers are added to the roll. You use your reaction to roll a 4 on the d8 and add it to the 19, causing the d20 to be treated as if it had rolled a 3.

Once this property of the ring has been used, it can't be used again until the next dawn.

Xanathar can take 3 legendary actions, choosing from the options below. He can take only one legendary action at a time and only at the end of another creature’s turn. The beholder regains spent legendary actions at the start of its turn.

Eye Ray. The beholder uses one random eye ray, rolling 1d20 to determine which eye to use.

Move. Xanathar can move with a fly speed of 30 ft.