Armor Class 16 (natural armor)
Hit Points 136 (16d12+32)
Speed 10 ft., swim 50 ft.
Saving Throws Str +10, Con +8
Skills Perception +4, Stealth +4
Damage Resistances cold
Senses blindsight 120 ft., passive Perception 14
Challenge 9 (5,000 XP)
Camouflage. The Lurker has advantage on Dexterity (Stealth) checks made to hide in water. Creatures that cannot see the Lurker before it attacks are surprised on a DC 16 Perception check.
Water Breathing. The Lurker can breathe only underwater.
Siege Monster. The Lurker deals double damage to objects and structures.
Hold Breath. The Lurker can hold its breath for 1 hour when pulled onto land.
Actions
Multiattack. The Lurker makes two attacks: one with its bite and one with its tentacle slam, or two tentacle slams.
Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 28 (4d10+6) piercing damage. If the target is a Large or smaller creature, it is grappled (escape DC 18). Until this grapple ends, the target is restrained and the Lurker can't bite another target.
Tentacle Slam. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 17 (2d10+6) bludgeoning damage and the target must succeed on a DC 18 Strength saving throw or be knocked prone.
Capsize (Recharge 5-6). The Lurker slams its body into a vessel it is adjacent to. Any vehicle of Large size or smaller capsizes. Crew and passengers must succeed on a DC 16 Dexterity saving throw or fall into the water.
Lore
The Deepwater Lurker is known to Saltmere sailors as the Greyshade -- a name that understates what it is. In open water, the creature is nearly invisible: its upper surface matches the grey of deep ocean, its underside the pale of sky, and its edges dissolve into the water in ways that the eye reads as current rather than creature. It can be twenty feet away before it is visible.
The Lurker hunts by patience. It finds the path of a ship, drops below it at depth, and follows for hours or days until an opportunity presents itself -- a crew member at the rail, a small boat lowered, a moment when the ship has slowed. Then it rises. Sailors describe the attack not as a monster appearing but as the sea opening and a piece of it reaching upward.
The Coastal Confederacy's shipping lanes between the Barrier Isles and Saltmere pass through the Lurker's primary range. The Harbor-Master's office keeps records of lost vessels that attribute cause to weather when the actual cause is more consistently directional than weather would produce.
Habitat
Deep coastal waters, submerged cave systems, the shipping lanes west of the Barrier Isles