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.  

Grappled

A grappled creature's speed becomes 0, and it can't benefit from any bonus to its speed. The condition ends if the grappler is incapacitated (see the condition). The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.   Mind Blast (Recharge 5–6). The elder brain magically emits psychic energy. 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.

Stunned

A stunned creature is incapacitated (see the condition), can't move, and can speak only falteringly. The creature automatically fails Strength and Dexterity saving throws. Attack rolls against the creature have advantage.

Psychic Link.

The elder brain targets one incapacitated creature it can perceive with its Creature Sense trait and establishes a psychic link with that creature. Until the psychic 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 psychic link, doing so with a successful DC 18 Charisma saving throw. On a successful save, the target takes 10 (3d6) psychic damage. The psychic link also ends if the target and the elder brain are more than 5 miles apart, with no consequences to the target. The elder brain can form psychic links with up to ten creatures at a time.

Incapacitated

An incapacitated creature can't take actions or reactions.

Sense Thoughts.

The elder brain targets a creature with which it has a psychic link. The elder brain gains insight into the target’s reasoning, its emotional state, and thoughts that loom large in its mind (including things the target worries about, loves, or hates). The elder brain can also make a Charisma (Deception) check with advantage to deceive the target’s mind into thinking it believes one idea or feels a particular emotion. The target contests this attempt with a Wisdom (Insight) check. If the elder brain succeeds, the mind believes the deception for 1 hour or until evidence of the lie is presented to the target.

Legendary Actions

The elder brain can take 3 legendary actions, choosing from the options below. It can take only one legendary action 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. Tentacle. The elder brain makes a tentacle attack.  

Break Concentration.

The elder brain targets a 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 1d4 psychic damage per level of the spell.  

Psychic Pulse.

The elder brain targets a creature within 120 feet of it with which it has a psychic link. Enemies of the elder brain within 10 feet of that creature take 10 (3d6) psychic damage.  

Sever Psychic Link.

The elder brain targets a 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.

Lair Actions

When fighting inside its lair, an elder brain can use lair actions. On initiative count 20 (losing initiative ties), an elder brain can take one lair action to cause one of the following effects; the elder brain can’t use the same lair action two rounds in a row:

• The elder brain casts wall of force.
• 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. If the target doesn’t or can’t use this benefit in that time, the inspiration is lost.
• 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 succeed on a DC 18 Charisma saving throw or be unable to leave its current space. It can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

The lair of an elder brain always lies deep in the heart of a mind flayer colony. The creature dwells in a dimly glowing brine pool, filled with foul and brackish water infused with the elder brain’s vital fluids and with psionic energy.

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:

• Creatures within 5 miles of an elder brain feel as if they are being followed, even when they are not.
• The elder brain can overhear any telepathic conversation happening within 5 miles of it. The creature that initiated the telepathic conversation makes a DC 18 Wisdom (Insight) check when telepathic contact is first established. If the check succeeds, the creature is aware that something is eavesdropping on the conversation. The nature of the eavesdropper isn’t revealed, and the elder brain can’t participate in the telepathic conversation unless it has formed a psychic link with the creature that initiated it.
• 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.

If the elder brain dies, these effects immediately end.