Siege Monster. The deep dweller deals double damage to objects and structures.

Water Breathing. The deep dweller can breathe only underwater.

Living Dungeon. Swallowed creatures become trapped inside the Deep Dweller's labyrinthine internal cavities and must survive the natural hazards within, such as pools of acid and parasitic chuuls.

Multiattack. The deep dweller makes three tentacle attacks, each of which it can replace with one use of Fling or Submerge.

Tentacle. Melee Weapon Attack: +9 to hit, reach 30 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. The deep dweller has ten tentacles, each of which can grapple one target. A tentacle is severed if it receives 15 or more slashing damage on a single turn. 

Fling. One Large or smaller object held or creature grappled by the deep dweller 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.

Submerge. A Large or smaller creature that is in the water and grappled by the deep dweller is dragged 60 feet underwater. If the creature drops to 0 hit points, or is dragged all the way to the depths, it is automatically swallowed, and the grapple ends. While swallowed, the creature has total cover against attacks and other effects outside the deep dweller.

If the deep dweller takes 30 damage or more on a single turn from a creature inside it, it must succeed on a DC 20 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 deep dweller. If the deep dweller dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

The deep dweller 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 deep dweller regains spent legendary actions at the start of its turn.

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

Mucus Cloud (Costs 3 Actions). While underwater, the deep dweller expels a mucus cloud in a 60-foot radius. The mucus spreads around corners, and that area is heavily obscured to creatures other than the deep dweller. Each creature other than the deep dweller that ends its turn there must succeed on a DC 15 Constitution saving throw, taking 16 (3d10) acid damage on a failed save, or half as much damage on a successful one. A strong current disperses the cloud, which otherwise disappears at the end of the deep dweller's next turn.