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.

Actions

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.

Legendary Actions

The elder brain 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 elder brain 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.

Elder Brain Dragon