Hunter Shark

A hunter shark on the party's side is a triton druid's wild shape, a sea-witch's familiar, or a sahuagin guide. There's always a handler, charm spell, or Beast Master ranger between the party and the shark. Intelligence 1 means no negotiation.

Use the shark as a single tool. Scout a wreck, patrol a reef, intercept a fleeing enemy across deep water. Swim 40 feet and Blindsight 60 feet make it the best underwater scout the party has. On land, it has walk speed 5 feet and is 45 HP of dying fish. Never bring it above water.

In combat, it's a positioning piece. One Bite at +6 for 3d6+4 per turn on a chosen target. It occupies water space enemies don't want to cross. Sahuagin and other sharks recognize it as kin and don't attack without orders.

The shark obeys while the spell or bond holds. Once the bond drops, the shark is hungry and the handler is the most appetizing creature in the water. Stage the alliance underwater in darkness. The shark visible only as a shadow at torch's edge. Sharks are scarier allied than hostile because the party knows the cost if the leash slips.

A hunter shark is a CR 2 underwater predator with 45 HP, AC 12, Blindsight 60 feet, Strength 18, and a Bite at +6 for 3d6+4 piercing. It has Advantage if the target isn't at full HP. The shark exists in water only; walk speed 5 feet means the party never fights it on land unless something went very wrong.

The encounter is the water, not the shark. Stage it during a shipwreck, a drowned temple, or a cliff where a PC was pushed into the sea. The party already fights the ocean, cold, and darkness. The shark is the punctuation mark. Blindsight 60 feet makes it untrackable in murk. It always knows where the party is even when they can't see 10 feet.

When the shark bites, it's a single attack per turn at +6 for 3d6+4 piercing, then disengages into darkness to circle for another pass. Advantage means the second bite of the round hits harder. Hunter sharks with Intelligence 1 don't retreat unless badly wounded. They're eating machines. If a PC reaches 0 HP in the water, the shark drags the body. Recovery requires a swim-down grapple.

Run the encounter in fragments. A fin breaks the water. Silence. Then a Dex save to avoid being knocked off the rope. The terror is what isn't visible.