Amphibious. Abyss can breathe air and water.

Freedom of Movement. Abyss ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from nonmagical restraints or being grappled.

Siege Monster. Abyss deals double damage to objects and structures.

Multiattack. Abyss makes three abyssal appendage attacks, each of which it can replace with one use of Fling or Void Maw.

Void Maw. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 23 (3d8 + 10) necrotic damage. If the target is a Large or smaller creature grappled by Abyss, 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 Abyss, and it takes 42 (12d6) psychic damage at the start of each of Abyss's turns. If Abyss takes 50 damage or more on a single turn from a creature inside it, Abyss 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 Abyss. If Abyss dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Abyssal Appendage. Melee Weapon Attack: +17 to hit, reach 30 ft., one target. Hit: 20 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. Abyss has ten appendages, each of which can grapple one target.

Fling. One Large or smaller object held or creature grappled by Abyss is thrown up to 60 feet in a random direction 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.

Sanity Blast. Abyss magically creates three bolts of psychic energy, each of which can strike a target Abyss can see within 120 feet of it. A target must make a DC 23 Wisdom saving throw, taking 22 (4d10) psychic damage on a failed save, or half as much damage on a successful one.

Abyss 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. Abyss regains spent legendary actions at the start of its turn.

Abyssal Appendage Attack or Fling (Costs 1 Action). Abyss makes one abyssal appendage attack or uses its Fling.

Sanity Blast (Costs 2 Actions). Abyss uses Sanity Blast.

Void Plume (Costs 3 Actions). While within its dark domain, Abyss expels a plume of shimmering void energy in a 60-foot radius. The cloud spreads around corners, and that area is heavily obscured to creatures other than Abyss. Each creature other than Abyss that ends its turn there must succeed on a DC 23 Wisdom saving throw, taking 16 (3d10) psychic damage on a failed save, or half as much damage on a successful one. A strong current or bright light disperses the plume, which otherwise disappears at the end of Abyss's next turn.