Brinemaw

Brine Sense. The brinemaw has blindsight while touching saltwater, brine, or salt-encrusted terrain. Creatures that are bleeding or have taken necrotic damage are automatically detected within 60 feet.

Salt-Dead Regeneration. The brinemaw regains 10 hit points at the start of its turn if it is in contact with salt, brine, or seawater. If the brinemaw takes radiant damage or is submerged in fresh water, this trait doesn’t function until the start of its next turn.

Colossal Bulk. The brinemaw can move through nonmagical structures, ships, and ruined walls as difficult terrain. Each creature of Medium size or smaller within 5 feet of Brinemaw when it moves must succeed on a DC 16 Strength saving throw or be knocked prone. 

Siege Mount. While a rider is mounted on the brinemaw, the brinemaw has advantage on saving throws against being stunned or restrained.

Corrosive Crystallization. Terrain within 15 feet of the Brinemaw begins to excrete corrosive salt crystals. Touching these crystals causes 1d6 acid damage.

 Multiattack. The brinemaw makes three Tentacle attacks, or two Tentacle attacks and one Bite attack.

Tentacle. Melee Attack Roll: +10, reach 15 ft. Hit: 15 (2d8 + 6) Bludgeoning damage. If the target is a Large or smaller creature, it has the Grappled condition (escape DC 16) from one of three tentacles. If the target is already grappled by the brinemaw, it is restrained instead.

Bite. Melee Attack Roll: +10, reach 5 ft. Hit: 27 (3d12 + 6) Piercing damage. If the target is a Large or smaller creature, it has the Grappled condition (escape DC 16). While Grappled, the target has the Restrained condition and can’t be targeted by the brinemaw's tentacles.

Salt Spray (Recharge 5–6). The brinemaw exhales a blast of pressurized salt water and salt crystal shards in a 30-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw, taking : 31 (7d8) acid damage on a failed save, or half as much on a success. On a failed save, a creature is also Blinded until the end of its next turn. 

Undertow Drag. The brinemaw pulls one creature it is grappling up to 15 feet closer to itself. If this movement pulls the creature into hazardous terrain (brine pools, rubble, collapsing ruins), the creature takes 7 (2d6) bludgeoning damage.

Brinemaw can take 2 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn.

Tentacle Lash. The brinemaw makes one Tentacle attack.

Salt Wake. Brinemaw moves up to half its speed without provoking opportunity attacks. The ground it moves through becomes difficult terrain until the start of Brinemaw’s next turn.