SEMPITERNAL, THE ELDEST BRAIN

My PCs have conquered the bad guys in “A Darkness Gathering” and now head off to the North in “Masters of Eternal Night“.  

There is a very cool encounter with an undead elder brain that, is literally, the eldest brain, because it was around during the time of the illithid empire.  The players can fight Sempiternal (along with its illithid vampire bodyguards) or negotiate.  My PCs negotiated. After they saw the stat block of the illithid vampires – as they got one as a temporary ally – they were VERY HAPPY they didn’t fight Sempiternal and his servants.

Not only did they secure a vampire illithid as a (dangerous and unpredictable) ally they explained to Sempiternal that they were being chased by a gang of mind flayers known as the Thralltakers (who had been reinforced by ancient mind flayers the PCs had left alive in the ship).  

MOST ELDER BRAIN

The entire area is made of rotted flesh and tendons. When the players approach the pool of Sempiternal they notice (on a Perception DC 23) ghostly tadpolelikeCreatures swimming throughout the pool.

Aberrant Walls: Illithids sculpted the walls to emanate an aberrant aura. Non-aberrant creatures in the room take a –2 penalty to saving throws.

Brain Pool: Sempiternal ate all the illithid tadpoles’ eons ago when he was still hoping for rescue. Ghostly echoes of these mind flayer tadpoles seethe in the now dry the brain. These spectral illithid infants magnify psychic trauma. Non-aberrant creatures have vulnerability to psychic while in the pool. A creature that ends its turn in the pool is subject to the following attack as ethereal young illithids swarm in a phantom hunger frenzy.

Feeding Frenzy. Melee Weapon Attack: auto hit, reach pool area, one target.
Hit: 26 (4d6 + 12) necrotic damage and the target is slowed (as the slow spell, DC 15 Con save ends)

• First Failed Saving Throw: The target is also dazed (DC 15 Con save ends both).
• Second Failed Saving Throw: The target is dominated by the Sempiternal, the elder brain (DC 15 Int or Wis save ends).

Caustic Goo: Caustic bodily fluid continues to ooze from a wound in the roof here surrounding the area immediately around Sempiternal’s pool. Any creature that enters or starts its turn in the goo takes 15 acid damage. A creature can take this damage only once per turn. Any creature that comes in contact with the goo glows for 5 minutes, taking a –10 penalty to Stealth checks.

TACTICS
If attacked, the elder brain telepathically commands the illithid vampires to emerge from their receptacles. The elder brain begins combat using mind burst, then uses sensory overload to teleport enemies into the brain pool. Whenever possible, it uses tentacle warp to place enemies within the reach of the illithid vampires. If the illithid vampire stuns a foe, that power instead teleports the enemy into the brain pool. When the elder brain sends a telepathic command, the illithid vampires enter from behind and attack any enemy within reach.

TREASURE
Brainmate lobe.

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SEMPITERNAL, UNDEAD ELDER BRAIN
Large undead (abberation), lawful evil
Armor Class 10
Hit Points 210 (20d10 + 100)
Speed 5 ft., swim 10 ft.

Saving Throws INT +10, WIS +9, CHA +12
Damage Resistances lightning, necrotic; bludgeoning, piercing, and slashing from Nonmagical Attacks that are not cold iron or silver
Condition Immunities immobilization, paralysis
Skills Arcana +10, Deception +12, Insight +14, Intimidation +12, Persuasion +12
Senses Blindsight 120 ft., Passive Perception 14
Languages Common, Deep Speech, Telepathy 5 miles, Undercommon understands but can’t speak
Challenge 14 (11,500 XP)

Absorb Thoughts. Whenever an ally within its Sempiternal’s lair drops to 0 hit points, the elder brain makes a saving throw or a power recharge roll.
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.
Regeneration. The elder brain regains 10 hit points at the start of its turn if it has at least 1 hit point and isn’t in sunlight. If the elder brain takes radiant damage or damage from holy water, this trait doesn’t function at the start of the elder brain’s next turn.
Sunlight Hypersensitivity. The elder brain takes 20 radiant damage when it starts its turn in sunlight. While in sunlight, it has disadvantage on attack rolls and ability checks.
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.
Thought Disruption. The connection the undead elder brain has to the Negative Energy Plane is charged with alien psychic energy that is antithetical to the living minds of most life forms. A creature that willfully touches the elder brain or its brackish brine pool or is struck by its tentacle attack gains 1 level of exhaustion. This exhaustion may be cancelled by sacrificing a sanity point.
Warp Reality.Any enemy that teleports while within Sempiternal’s lair takes 10 psychic damage.

ACTIONS
Flailing Warp. Sempiternal uses sensory overload or mind blast (if available but not on the same round) and then uses tentacle warp three times, making each attack against a different target.

Tentacle Warp. Melee Weapon Attack: +7 to hit, reach 30 ft., one target.
Hit: 20 (4d8 + 2) necrotic damage plus thought disruption. The elder brain teleports the target 15 feet, and the target is immobilized until the end of the elder brain’s next turn.

Mind Blast (Recharge 5–6). Sempiternalmanifests psychic energy targeting creatures of it’s choice within 60 feet of it and take 32 (5d10 + 5) psychic damage and must succeed on a DC 18 Intelligence saving throw or 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.

Sensory Overload (Recharge 5–6). Sempiternal fills the space surround itself out to 25 feet (sphere) with psionically empowered visual and audio sensory feedback. Each target takes 15 (2d8+5) psychic damage and must make a DC 18 Intelligence saving throw or become blinded and deafened until the end of the elder brain’s next turn. Pass or fail the elder brain can teleport affected target up to 15 feet.

REACTIONS
Baleful Transfer. When Sempiternalsaves against an effect an enemy within its lair is subjected to the same effect the elder brain saved against with the same conditions.

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 warp 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.