Ancient Beholder

Aberrant Willpower (Recharges after a Short or Long Rest). If the beholder would be reduced to 0 hit points, its current hit point total instead resets to 280 hit points, and it regains any expended uses of Legendary Resistance. Additionally, the beholder now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 25,000 XP (50,000 XP total) for defeating the beholder after its Aberrant Willpower activates.

Legendary Resistance (4/Day). If the beholder fails a saving throw, it can choose to succeed instead.

Magic Resistance. The beholder has Advantage on saving throws against spells and other magical effects.

Multiattack. The beholder uses Eye Rays four times.

Bite. Melee Attack Roll: +10, reach 5 ft. Hit: 20 (4d6 + 4) Piercing damage.

Eye Rays. The beholder randomly shoots one of the following magical rays at a target it can see within 150 feet of itself (roll 1d20; reroll if the beholder has already used that ray during this turn):

1: Charm Ray. Wisdom Saving Throw: DC 18. Failure: 18 (4d8) Psychic damage, and the target has the Charmed condition for 1 hour or until it takes damage. Success: Half damage only.
2: Blinding Ray. Constitution Saving Throw: DC 18. Failure: The target has the Blinded 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: Paralyzing Ray. Constitution Saving Throw: DC 18. 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.
4: Fear Ray. Wisdom Saving Throw: DC 18. Failure: 18 (4d8) Psychic damage, and the target has the Frightened condition until the end of its next turn.
5: Incapacitating Ray. Wisdom Saving Throw: DC 18. Failure: The target has the Incapacitated 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.
6: Enfeebling Ray. Constitution Saving Throw: DC 18. Failure: 14 (4d6) Necrotic damage, and the target has Disadvantage on Strength-based D20 Tests until the end of its next turn. Success: Half damage only.
7: Slowing Ray. Constitution Saving Throw: DC 18. Failure: 21 (6d6) Necrotic damage. Until the end of the target’s next turn, the target’s Speed is quartered; 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.
8: Enervation Ray. Constitution Saving Throw: DC 18. Failure: 18 (4d8) 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.
9: Repulsion Ray. Strength Saving Throw: DC 18. Failure: The target is pushed up to 30 feet straight away from the beholder and has the Prone condition.
10: Telekinetic Ray. Strength Saving Throw: DC 18 (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.
11: Deafening Ray. Constitution Saving Throw: DC 18. Failure: 18 (4d8) Thunder damage. The target has the Deafened 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. Success: Half damage only.
12: Stun Ray. Constitution Saving Throw: DC 18. Failure: The target has the Stunned 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.
13: Sleep Ray. Wisdom Saving Throw: DC 18 (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.
14: Exhaustion Ray. Constitution Saving Throw: DC 18. Failure: 14 (4d6) Necrotic damage and the target gains 1 Exhaustion level. Success: Half damage only.
15: Petrification Ray. Constitution Saving Throw: DC 18. 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.
16: Compulsion Ray. Wisdom Saving Throw: DC 18. Failure: 18 (4d8) Psychic damage, and the target uses its Reaction to move up to its speed in a direction of the beholder's choice. Success: Half damage only.
17: Banishing Ray. Charisma Saving Throw: DC 18. Failure: The target is banished to a harmless demiplane 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.
18: Befuddling Ray. Intelligence Saving Throw: DC 18. Failure: (4d6) Psychic damage, and the target can't take the Magic action until the end of its next turn. Success: Half damage only.
19: Disintegration Ray. Dexterity Saving Throw: DC 18. Failure: 36 (8d8) 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.
20: Death Ray. Dexterity Saving Throw: DC 18. Failure: 55 (10d10) Necrotic damage. Success: Half damage. Failure or Success: The target dies if the ray reduces it to 0 Hit Points.

Antimagic Cone. The beholder’s central eye emits an antimagic wave in a 200-foot Cone. Until the start of the beholder’s next turn, that area acts as an Antimagic Field spell, and that area works against all beholder Eye Rays.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the beholder can expend a use to take one of the following actions. The beholder regains all expended uses at the start of each of its turns.

Chomp. The beholder makes three Bite attacks.

Glare. The beholder uses Eye Rays.

Slime Cube. Dexterity Saving Throw: DC 18, each creature in a 25-foot-radius Cube centered on a point the beholder can see within 120 feet. Failure: 18 (4d8) Acid damage, and the target has the Restrained condition. Success: Half damage. Failure or Success: The area is Difficult Terrain until the start of the beholder's next turn. The beholder can't take this action again until the start of its next turn.

If the beholder's Aberrant Willpower has activated in the last hour, it can use the options below as Legendary Actions.

Blink. The beholder uses Antimagic Cone.

Flee. The beholder moves up to its Speed without provoking Opportunity Attacks. The beholder can’t take this action again until the start of its next turn.