Reef Razor
Traits
Water Breathing. Can breathe only underwater.
Blood Frenzy. The Reef Razor has Advantage on attacks against a creature that isn't at full HP.
Ripping Pass. If the Reef Razor moved at least 40 feet this turn, its attacks deal an extra 10 (3d6) Slashing damage.
Slipstream. The Reef Razor's movement through water doesn't provoke Opportunity Attacks. (A free Disengage; it also ignores water currents that count as Difficult Terrain.)
Beached. When out of water, the Reef Razor's Speed is 10 feet, its attack rolls have Disadvantage, and Ripping Pass and Slipstream don't function. If it stays out of water for more than 1 minute, it begins to suffocate.
Actions
Multiattack. Makes one Bite and one Razor Fin attack.
Bite. Melee Attack Roll: +7, reach 5 ft. Hit: 15 (2d10 + 4) Piercing damage. If the target is Large or smaller and the Reef Razor doesn't already have a target grappled, the target has the Grappled condition (escape DC 15). The Reef Razor can drag a grappled target while moving.
Razor Fin. Melee Attack Roll: +7, reach 10 ft. Hit: 13 (2d8 + 4) Slashing damage. If the Reef Razor moved 40 feet this turn, plus an extra 10 (3d6) Slashing (Ripping Pass).
Shredding Pass (Recharge 5–6). The Reef Razor moves 60 feet (swimming, in a straight line; doesn't provoke Opportunity Attacks). Every enemy within 5 feet of this line makes a DC 15 Dexterity save. Failure: 27 (6d8) Slashing damage, and it begins bleeding — the target takes 5 (2d4) damage at the start of each of its turns (stopped with an action and a DC 12 Wisdom (Medicine) check, or any healing). Success: Half damage, no bleeding.
Bonus Actions
Circle Back. The Reef Razor moves half its Swim Speed (30 feet). This movement adds to the distance counted for Ripping Pass.
CR Rationale: Defensive — AC 15 / HP 114, right above the CR 6 target; a single resistance (Cold) and the requirement to fight in water balance its fragility. Offensive — 15 + 13 + 10 = 38 damage per turn, with Blood Frenzy's Advantage effectively ~46; within the CR 6 band (~40/turn). Shredding Pass hits multiple targets but depends on Recharge. The bleed effect spreads damage over time. Save DC 15 matches the CR 6 norm.
How to Use
The teacher of underwater combat. Underwater encounters are usually dull for most parties because everyone is slow and no one can maneuver. The Reef Razor flips this: it swims 60 feet, the party moves 15.
Tactic: it never stops. It circles in a loop, dealing 38 damage on every pass (with Ripping Pass active), taking no Opportunity Attacks thanks to Slipstream. The party can't chase it — they can only wait.
Blood Frenzy is a pressure mechanism: once it hits someone, that person becomes a permanent Advantage target. The party is forced to play "protect the wounded."
Key: Get it out of the water — or take the water away from it.
The Beached trait is the heart of this fight. Out of water, the Reef Razor is: Speed 10, Disadvantage on attacks, no extra damage, no escape. It's finished.
How to do it: Thorn Whip, Telekinesis, Bigby's Hand, a strong Grapple plus dragging it ashore, Control Water (pulling the water back), or simply drawing the fight into shallow water / the shore / a ship's deck. If the party goes into deep water, they play the Reef Razor's game. If they pull it ashore, they make it play theirs.
Alternative: stop it. Ripping Pass depends on the 40-foot movement requirement. A narrow coral tunnel, a sunken ship's hold, a cave — anywhere without room to maneuver cuts its damage by a third.