Brain Dump. Whenever the greater neh-thalggu consumes a brain, it gains the magical ability to speak and understand languages known by the brain’s previous owner.
Unusual Nature. The greater neh-thalggu doesn’t require air.
Spellcasting (Psionics). The greater neh-thalggu casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 15). It must have consumed the requisite number of brains to cast the spell, as indicated:
Tier 1 (3/day each): arms of Hadar (1 brain), detect magic (2 brains), magic missile (3 brains), Tenser’s floating disk (4 brains),
Tier 2 (2/day each): darkness (5 brains), hold person (6 brains), invisibility (7 brains), spider climb (8 brains), fear (9 brains),
Tier 3 (1/day each): hypnotic pattern (10 brains), major image (11 brains), stinking cloud (12 brains) Greater Invisibility(13 brains)
Multiattack. The greater neh-thalggu makes one Bite attack and two Claw attacks.
Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 20 (4d8 + 4) piercing damage.
Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (4d4 + 2) slashing damage.
Extract Brain. Melee Weapon Attack: +4 to hit, reach 5 ft., one incapacitated Humanoid. Hit: 35 (10d6) piercing damage. If this damage reduces the target to 0 hit points, the greater neh-thalggu kills the target by extracting and consuming its brain.
Mind Blast (Recharge 5–6). The greater neh-thalggu magically emits psychic energy at all Humanoids it can see within 10 feet of itself. The target must make a DC 14 Wisdom saving throw. On a failed save, the target takes 9 (2d8) psychic damage and is incapacitated until the end of its next turn. On a successful save, the target takes half as much damage and isn’t incapacitated.
Of Many Minds (Costs 1 x Tier Actions): The greater neh-thalggu activates one of the spells granted to it by it's Spellcasting (Psionics) ability, provided it is able to use the spell.
Mind Blast (2 Actions): See Actions above, but the greater neh-thalggu can only target one creature.
Bite (2 Actions): See Actions above
Claw (1 Action): See Actions above
Description
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Lair and Lair Actions
Lair Actions
On initiative count 20 (losing initiative ties), the greater neh-thalggu takes a lair action to cause one of the following effects; the greater neh-thalggu can’t use the same effect two rounds in a row:
- Maw of Madness - The inhuman nature of the aberrations frays the minds of the adventurers. Each non-aberration that can see the aberrations must make a DC 10 intelligence saving throw. Each creature that fails, gains a short-term madness from the madness chart. (Credit to "DM's Box of Duct Tape")
Regional Effects
The region containing a legendary greater neth-thalggu’s lair is seeped in psychic cries of anguish, which creates one or more of the following effects:
- Anyone who sleeps within 1 mile of the lair suffers from night terrors, as the aberration imparts visions of madness and carnage into their dreams. On a failed DC 8 CON save, the creature does not gain the benefits of a long rest.
If the greater neh-thalggu dies, these effects fade over the course of 1d4 days.
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