Amalgamation

Legendary Resistance (3/Day). If the amalgamation fails a saving throw, it can choose to succeed instead.

Malignancy. The amalgamation can’t be teleported or travel to another plane against its will.

Immutable Form. The amalgamation is immune to any spell or effect that would alter its form.

Magic Weapons. The amalgamation’s weapon attacks are magical.

Innate Spellcasting. The amalgamation’s innate spellcasting ability is Charisma (spell save DC 20). It can innately cast the following spells, requiring no components:

At will: Detect Thoughts, Dispel Magic

Multiattack. The Amalgamation uses its Contaminating Presence. It
then makes three Tentacle attacks, each of which it can replace with
one use of Fling. 

Bite. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 23 (3d8 + 10) piercing damage. If the target is a Large or smaller creature grappled by the amalgamation, that creature is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the amalgamation, and it takes 42 (12d6) acid damage and gains one level of contamination at the start of each of the amalgamation’s turns. If the amalgamation takes 50 damage or more on a single turn from a creature inside it, the amalgamation must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the amalgamation. If the amalgamation dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Tentacle. Melee Weapon Attack: +17 to hit, reach 50 ft., one creature. Hit: 20 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. The amalgamation has twelve tentacles, each of which can grapple one target.

Fling. One Large or smaller object held or creature grappled by the amalgamation is thrown up to 60 feet in a direction of the amalgamation’s choice and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 18 Dexterity saving throw or take the same damage and be knocked prone.

Mind-breaking Whispers. The amalgamation utters a terrible truth within the mind of one creature within 120 feet of it. That creature must succeed on a DC 20 Intelligence saving throw. On a failure, it takes 35 (10d6) psychic damage and becomes insane until it finishes a long rest. While insane, it can’t take actions, can’t understand what other creatures say, can’t read, and can speak only in gibberish. A Greater Restoration spell cast on the creature ends this effect.

Contaminating Presence. Humanoid creatures within 120 feet of the amalgamation must succeed on a DC 20 Constitution saving throw or take 10 (3d6) necrotic damage and gain one level of contamination.

The amalgamation can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The amalgamation regains spent legendary actions at the start of its turn.

Tentacle Attack or Fling. The amalgamation makes one tentacle attack or uses its Fling.

Mind-breaking Whispers (Costs 2 Actions). The amalgamation uses Mind-breaking Whispers.

Piper at the Gate (Costs 3 Actions). Up to five gibbering mouther appear in unoccupied spaces within 30 feet of the amalgamation and remain until destroyed. These creatures act after the amalgamation on the same initiative count and are under its control. The amalgamation can have up to ten gibbering mouther summoned by this ability at a time.