Ancient Green Elder Brain Beholder Dragon

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

Siege Monster. The dragon deals double damage to objects and structures.

Unusual Nature. The dragon doesn’t require air or sleep.

Multiattack. The dragon makes one Bite attack, two Claw attacks, and one Tentacle attack.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 11 (2d10) psychic damage.

Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 11 (1d6 + 8) slashing damage.

Tentacle. Melee Weapon Attack: +15 to hit, reach 15 ft., one creature. Hit: 12 (1d8 + 8) psychic damage. If the target is Huge or smaller, it is grappled (escape DC 18). The dragon can have up to four targets grappled at a time.

Tadpole Brine Breath (Recharge 5–6). The dragon exhales brine in a 120-foot line that is 15 feet wide. Each creature in that area must make a DC 22 Constitution saving throw, taking 55 (10d10) psychic damage on a failed save, or half as much damage on a successful one. On a success or failure, if the creature isn’t a Construct or an Undead, it becomes infested with illithid tadpoles.

While infested, the creature takes 16 (3d10) psychic damage at the start of each of its turns. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself after it succeeds on three of these saves. If the creature is targeted by magic that ends a curse or restores 40 hit points or more, the tadpoles infesting the creature are killed instantly, ending the effect on the creature.

If a Humanoid is reduced to 0 hit points while infested, the creature is stable but remains unconscious for 6d12 hours. When the period of unconsciousness ends, the creature transforms into a mind flayer (see the Monster Manual) with all its hit points. Casting a wish spell on the unconscious creature rids it of the infestation and prevents it from turning into a mind flayer.

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

1. Charm Ray. The targeted creature must succeed on a DC 16 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 16 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 16 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 16 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 16 Constitution saving throw, taking 36 (8d8) 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 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 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 ft. 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 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 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 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 Dexterity saving throw or take 45 (10d8) 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 16 Dexterity saving throw or take 55 (10d10) necrotic damage. The target dies if the ray reduces it to 0 hit points.

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

Tentacle. The dragon makes one Tentacle attack.

Shatter Concentration (Costs 2 Actions). The dragon targets a creature it is grappling. The target’s concentration on a spell it has cast or an ability it is maintaining ends, and the target takes 19 (3d12) psychic damage.

Glare. The beholder uses Eye Rays.