Creature Sense.

The elder brain is aware of creatures within 5 miles of it that have an Intelligence score of 4 or higher. It knows the distance and direction to each creature, as well as each one's Intelligence score, but can't sense anything else about it. A creature protected by a mind blank spell, a nondetection spell, or similar magic can't be perceived in this manner.

Legendary Resistance (3/Day).

If the elder brain fails a saving throw, it can choose to succeed instead.

Magic Resistance.

The elder brain has advantage on saving throws against spells and other magical effects.

Telepathic Hub.

The elder brain can use its telepathy to initiate and maintain telepathic conversations with up to ten creatures at a time. The elder brain can let those creatures telepathically hear each other while connected in this way.

Actions

Tentacle.

Melee Weapon Attack: +7 to hit, reach 30 ft., one target. Hit: 20 (4d8 + 2) bludgeoning damage. If the target is a Huge or smaller creature, it is grappled (escape DC 15) and takes 9 (1d8 + 5) psychic damage at the start of each of its turns until the grapple ends. The elder brain can have up to four targets grappled at a time.

Mind Blast (Recharge 5–6).

Creatures of the elder brain's choice within 60 feet of it must succeed on a DC 18 Intelligence saving throw or take 32 (5d10 + 5) psychic damage and be stunned for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Spellcasting (Psionics).

The elder brain casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 18):

At will: detect thoughts, levitate
3/day: modify memory
1/day each: dominate monster, plane shift (self only)

Bonus Actions

Psychic Link.

The elder brain targets one incapacitated creature it senses with its Creature Sense trait and establishes a psychic link with the target. Until the link ends, the elder brain can perceive everything the target senses. The target becomes aware that something is linked to its mind once it is no longer incapacitated, and the elder brain can terminate the link at any time (no action required). The target can use an action on its turn to attempt to break the link, doing so with a successful DC 18 Charisma saving throw. On a successful save, the target takes 10 (3d6) psychic damage. The link also ends if the target and the elder brain are more than 5 miles apart. The elder brain can form psychic links with up to ten creatures at a time.

Sense Thoughts.

The elder brain targets a creature with which it has a psychic link. The elder brain gains insight into the target's emotional state and foremost thoughts (including worries, loves, and hates).

Legendary Actions

The elder brain 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 regains spent legendary actions at the start of its turn.

Break Concentration. The elder brain targets one creature within 120 feet of it with which it has a psychic link. The elder brain breaks the creature's concentration on a spell it has cast. The creature also takes 2 (1d4) psychic damage per level of the spell.
Psychic Pulse. The elder brain targets one creature within 120 feet of it with which it has a psychic link. The target and enemies of the elder brain within 30 feet of target take 10 (3d6) psychic damage.
Sever Psychic Link. The elder brain targets one creature within 120 feet of it with which it has a psychic link. The elder brain ends the link, causing the creature to have disadvantage on all ability checks, attack rolls, and saving throws until the end of the creature's next turn.
Tentacle (Costs 2 Actions). The elder brain makes one Tentacle attack.

Lair Actions

On initiative count 20 (losing initiative ties), an elder brain can take one of the following lair actions; the elder brain can't take the same lair action two rounds in a row:

Force Wall. The elder brain casts wall of force.
Psionic Anchor. The elder brain targets one creature it can sense within 120 feet of it and anchors it by sheer force of will. The target must make a DC 18 Charisma saving throw. On a failed save, its speed is reduced to 0, and it can't teleport. It can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Psychic Inspiration. The elder brain targets one friendly creature it can sense within 120 feet of it. The target has a flash of inspiration and gains advantage on one attack roll, ability check, or saving throw it makes before the end of its next turn.

Regional Effects

The territory within 5 miles of an elder brain is altered by the creature's psionic presence, which creates one or more of the following effects:

Paranoia. Creatures within 5 miles of an elder brain feel as if they are being followed, even when they're not.
Psychic Whispers. Any creature with which the elder brain has formed a psychic link hears faint, incomprehensible whispers in the deepest recesses of its mind. This psychic detritus consists of the elder brain's stray thoughts commingled with those of other creatures to which it is linked.
Telepathic Eavesdropping. The elder brain can overhear any telepathic conversation within 5 miles of it. The creature that initiated the telepathic conversation makes a DC 18 Wisdom saving throw when telepathic contact is first established. If the save is successful, the creature is aware that something is eavesdropping. The nature of the eavesdropper isn't revealed.

If the elder brain dies, these effects immediately end.

Environment: Underdark

Elder Brain