Nhaal-Uthys, the Trench That Dreams

gargantuan aberration, Chaotic Neutral · CR 27

by Deepsounding

Armor Class: 22

Hit Points: 656

Speed: 20 ft., fly 40 ft., swim 120 ft.

Saving Throws: strength, constitution, intelligence, wisdom

Skills: Arcana +16, History +16, Insight +15, Perception +15

Damage Resistances: Bludgeoning (nonmagical), Fire, Lightning, Piercing (nonmagical), Slashing (nonmagical), Thunder

Damage Immunities: Cold, Poison, Psychic

Condition Immunities: Blinded, Charmed, Deafened, Frightened, Paralyzed, Petrified, Poisoned, Prone, Stunned

Senses: darkvision 120 ft., tremorsense 120 ft., truesight 120 ft., passive Perception 25

Languages: Deep Speech, Primordial, Aquan

Challenge: 27 (105,000 XP)

STR 30 (+10)

DEX 14 (+2)

CON 30 (+10)

INT 27 (+8)

WIS 25 (+7)

CHA 26 (+8)

Traits

Amphibious. Nhaal-Uthys can breathe air and water.
Legendary Resistance (3/Day). If Nhaal-Uthys fails a saving throw, it can choose to succeed instead.
Magic Resistance. Nhaal-Uthys has advantage on saving throws against spells and other magical effects.
Dreaming Depth. Nhaal-Uthys is never fully awake. Any creature that finishes a long rest within 20 miles of it must succeed on a DC 26 Wisdom saving throw or gain no benefit from that rest, having spent the night inside a fragment of the trench's dream. On a failed save by 5 or more, the creature also learns one true thing it did not want to know.
Crushing Presence. The water around Nhaal-Uthys carries its weight. A creature that starts its turn within 60 feet of it takes 22 (4d10) bludgeoning damage and has its speed halved until the start of its next turn unless it is immune to being grappled.

Multiattack. Nhaal-Uthys makes one Sounding Cry attack and three Tendril attacks, or it makes four Tendril attacks.
Tendril. Melee Weapon Attack: +19 to hit, reach 40 ft., one target. Hit: 6d10 + 10 bludgeoning damage. The target is grappled (escape DC 27) if it is a Huge or smaller creature, and it is restrained until the grapple ends.
Sounding Cry (Recharge 5-6). Hit: 16d8 thunder damage. The target must succeed on a DC 26 Constitution saving throw or be deafened, stunned for 1 minute. It takes half as much damage on a successful one. Nhaal-Uthys sounds a single note that the water carries for a hundred miles. Each creature within 120 feet of it must make the saving throw.
Draw Down. Nhaal-Uthys pulls one creature it is grappling into the trench. The creature descends 200 feet, takes 44 (8d10) cold damage from the pressure, and must succeed on a DC 26 Constitution saving throw or fall unconscious for 1 minute. A creature that begins its turn below this depth repeats the damage.

Sink. Nhaal-Uthys descends up to 60 feet without provoking opportunity attacks. Any creature it is grappling descends with it.

The Trench Answers. Nhaal-Uthys dreams the spell differently. The caster must succeed on a DC 26 Charisma saving throw or the spell fails and its slot is expended, and the caster takes 27 (6d8) psychic damage.

Tendril.
Undertow. Nhaal-Uthys drags the water. Each creature within 60 feet must succeed on a DC 26 Strength saving throw or be pulled up to 40 feet toward it and knocked prone.
A Dream of Drowning. One creature Nhaal-Uthys can see within 120 feet dreams, briefly, that it has always been underwater. It must succeed on a DC 26 Wisdom saving throw or begin suffocating as though it had no air for 1 minute, repeating the save at the end of each of its turns.

Lair Action 1. The trench exhales. A column of freezing water 30 feet wide erupts from the seabed at a point Nhaal-Uthys can see within 120 feet; each creature in it must succeed on a DC 26 Constitution saving throw or take 27 (6d8) cold damage and be pushed 20 feet upward.
Lair Action 2. The pressure changes without warning. Each creature within 120 feet of Nhaal-Uthys must succeed on a DC 26 Constitution saving throw or be deafened and have disadvantage on Wisdom (Perception) checks until initiative count 20 on the following round.

Regional Effect 1. Within 20 miles of the trench, the sea does not reflect the sky. Navigators lose the horizon at dusk and cannot fix a position by star or sun until they leave the region.
Regional Effect 2. Fish within 20 miles of the trench swim in slow, deliberate patterns that observers describe as writing. Any creature that watches them for 10 minutes must succeed on a DC 20 Wisdom saving throw or be compelled to travel toward the trench for the next hour.