Legendary Resistance (3/Day, or 5/Day in Lair). If the elder orb fails a saving throw, it can choose to succeed instead.

Magic resistance. The elder orb has advantage on saving throws against spells and magical effects.

Control Death Tyrant. Once per day the elder orb can take control of a Death Tyrant. This power works as per the Dominate Monster spell.

Create Death Tyrant. Once per day the elder orb can create a Death Tyrant from the corpse of a beholder. This power works as per the Animate Dead, but only work on dead beholders and only create one Death Tyrant.

Spellcasting. The elder orb casts one of the following spells, using Intelligence as the spellcasting ability (spell save DC 20):

At Will: Acid Arrow, Detect Magic, Detect Thoughts, Magic Missile (level 3 version), Major Image

2/Day Each: Arcane Eye, Fireball (level 5 version), Cone of Cold, Invisibility, Mirror Image

1/Day Each: Disintegrate, Dream, Power Word Stun, Prismatic Spray, Reverse Gravity, True Seeing 

Multiattack. The elder uses Eye Rays three times.

Bite. Melee Attack Roll: +10, reach 5 ft. Hit: 20 (5d6 + 3) Piercing damage.

Eye Rays. The elder orb randomly shoots one of the following magical rays at a target it can see within 120 feet of itself (roll 1d10; reroll if the elder orb has already used that ray during this turn):

1: Charm Ray. Wisdom Saving Throw: DC 20. Failure: 22 (5d8) Psychic damage, and the target has the Charmed condition for 1 hour or until it takes damage. Success: Half damage only.
2: Paralyzing Ray. Constitution Saving Throw: DC 20. Failure: The target has the Paralyzed condition and repeats the save at the end of each of its turns, ending the effect on itself on a success. After 1 minute, it succeeds automatically.
3: Fear Ray. Wisdom Saving Throw: DC 20. Failure: 21 (6d6) Psychic damage, and the target has the Frightened condition until the end of its next turn.
4: Slowing Ray. Constitution Saving Throw: DC 20. Failure: 21 (6d8) Necrotic damage. Until the end of the target’s next turn, the target’s Speed is halved; the target can’t take Reactions; and it can take either an action or a Bonus Action on its turn, not both. Success: Half damage only.
5: Enervation Ray. Constitution Saving Throw: DC 20  Failure: 22 (5d8) Poison damage, and the target has the Poisoned condition until the end of its next turn. While Poisoned, the target can’t regain Hit Points. Success: Half damage only.
6: Telekinetic Ray. Strength Saving Throw: DC 20 (the target succeeds automatically if it is Gargantuan). Failure: The beholder moves the target up to 30 feet in any direction. The target has the Restrained condition until the start of the beholder’s next turn or until the beholder has the Incapacitated condition. The beholder can also exert fine control on objects with this ray, such as manipulating a tool or opening a door or container.
7: Sleep Ray. Wisdom Saving Throw: DC 20 (the target succeeds automatically if it is a Construct or an Undead). Failure: The target has the Unconscious condition for 1 minute. The condition ends if the target takes damage or a creature within 5 feet of it takes an action to wake it.
8: Petrification Ray. Constitution Saving Throw: DC 20. First Failure: The target has the Restrained condition and repeats the save at the end of its next turn if it is still Restrained, ending the effect on itself on a success. Second Failure: The target has the Petrified condition instead of the Restrained condition.
9: Disintegration Ray. Dexterity Saving Throw: DC 20. Failure: 45 (10d8) Force damage. If the target is a nonmagical object or a creation of magical force, a 10- foot Cube of it disintegrates into dust. Success: Half damage. Failure or Success: If the target is a creature and this damage reduces it to 0 Hit Points, it disintegrates into dust.
10: Death Ray. Dexterity Saving Throw: DC 20. Failure: 66 (12d10) Necrotic damage. Success: Half damage. Failure or Success: The target dies if the ray reduces it to 0 Hit Points.

Antimagic Cone. The elder orb’s central eye emits an antimagic wave in a 150-foot Cone. Until the start of the beholder’s next turn, that area acts as an Antimagic Field spell, and that area works against the beholder’s own Eye Rays.

2/Day Each: Shield, Counterspell

Legendary Action Uses: 3 (5 in Lair). Immediately after another creature’s turn, the elder orb can expend a use to take one of the following actions. The elder orb regains all expended uses at the start of each of its turns.

Chomp. The elder orb makes two Bite attacks.

Glare. The elder orb uses Eye Rays.

Quick cast (1 action for at will, 2 actions for 2/day, 3 actions for 1/day). The elder orb uses it's spell casting ability.