Gargathane

A strange Spirit who steals the souls from those that it devours; it then uses these souls to corrupt others or make unwilling puppets. These puppets worship it like a god-like being, referring to it as "The Master". These puppets work to gather more people to it so that it can devour them.
The last sighting of Gargathan was in the lands near the Elven Woods but since then he has not been seen. The Elves are on guard to fight him due to the damage he can do to any society that he is around.

Physical Description

General Physical Condition

A horrifying creature that seems to resemble a floating fish or singular head. It has tendrils emerging from all over its body. The two longest ones, attached to its jaws, end in wicked hooks that drag its victims to its mouth.

Gargathane CR: 8

Large fiend, typically neutral evil

Armor Class: 15 (natural armor)

Hit Points: 120 (16d10 + 32)

Speed: 30 ft , swim: 30 ft , can hover

STR

10 +0

DEX

8 -1

CON

14 +2

INT

16 +3

WIS

17 +3

CHA

14 +2

Skills: Investigation +3, Perception +9

Damage Resistances: Cold, Fire, Lightning; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks

Damage Immunities: Poison

Condition Immunities: Poisoned

Senses: Truesight 120 ft., Passive Perception 19

Languages: Abyssal, Common, Infernal, Telepathy 60 ft.

Challenge Rating: 8 ( 3,900 XP)

Proficiency Bonus: +3

Gargathane casts one of the following spells, using Intelligence as the spellcasting ability (spell save DC 14):

2/day: blur, charm person, crown of madness, phantasmal killer

Illusions of the Dead. Any creature that Gargathane eats has its soul devoured by the creature. It can then use the spell Simulacrum to create a copy of any creature it has devoured. Gargathane can use a living creature that it has charmed to anchor this soul. When it does, the living creature is unaware of the possession and Gargathane can see through its senses and knows where the living creature is at all times. When the living creature goes to sleep, the soul gains possession of the body.
Magic Resistance. Gargathane has advantage on saving throws against spells and other magical effects.

Actions

Multiattack. Gargathane makes one Bite and one Tendril attack.
Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 6 (2d6) piercing damage plus If the target is a medium or smaller creature, it must succeed on a DC 15 Dexterity saving throw or be swallowed by Gargathane. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the Gargathane, and it takes 9 (3d6) psychic damage at the start of each of Gargathane's turns. They can only have one creature swallowed at one time.
If Gargathane takes 20 damage or more on a single turn from a creature inside it, they must succeed on a DC 16 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of them. If Gargathane dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.
Tendril. Melee Weapon Attack: +3 to hit, reach 15 ft., one target. Hit: 3 (1d6) bludgeoning damage plus 9 (2d8) force damage. If the target is Medium or smaller, it is grappled (escape DC 15), pulled up to 30 feet toward Gargathane, and restrained until the grapple ends. Gargathane can grapple one target at a time with its tendril.