Giant Crab

A giant crab on the party's side is a stretch, but a sahuagin scout's pet, a sea-witch's familiar substitute (the find familiar list reads "spider, crab, hawk", so this is the upsized cousin), or a tide-pool oracle's bonded servant all make the framing work. Pick one and let the crab clack along behind the relevant PC like a barnacle-crusted dog.

In combat, the crab does almost nothing. The +3 to hit and 13 HP mean it dies to a stiff breeze and barely scratches anyone above CR 1. Its real value is sensory and atmospheric. Blindsight 30 ft. inside its perception bubble means it notices invisible attackers, things hiding under floors, and creatures the rogue's Perception missed. Stealth +3 plus the Swim speed lets it scout a flooded ruin or a harbor without being seen by anything on the surface. Treat it as a remote camera that occasionally pinches.

Out of combat, the crab is comic relief and a small loyalty token. It eats anything dead, it sleeps in the kettle, it once saved the cleric's holy symbol from rolling into the bilge. Players will name it within twenty minutes of meeting it, and if you kill the crab without warning, you have made a permanent enemy at the table. Use that responsibly. Crabs die only off-screen and only between sessions.

If the crab does something useful, like spotting a hidden sahuagin in the shallows, narrate the click of the claws before the player rolls Perception. The sound is the heads-up.

A giant crab is a beach hazard at CR 1/8. The SRD 5.2 stat block here has no listed actions, so run the claw as a Strength-based unarmed strike at +3 to hit for around 1d6+1 bludgeoning, or grant the older edition's claw with a free grapple on a hit if your group prefers. With AC 15 and only 13 HP, the crab pops on the first solid hit, so this is a swarm-style encounter or a low-stakes hazard rather than a real fight. Run two to six of them at once, scuttling out of tide pools, mangroves, or shipwreck holds.

The hook is Blindsight 30 ft. and Stealth +3, both inside that 30 ft. radius the crab perceives perfectly. The crab is buried in sand or wedged behind rock until the party steps within range, and then it is suddenly there. Use that. Don't roll initiative until a PC has stepped on the first crab. The reveal is the encounter. After that, the crabs swing once, get hit once, and pop, and the party walks on with one more bandage used up.

The Swim 30 ft. and amphibious lifestyle mean the crab fights as well in tidewater as on the beach. If the party tries to retreat into the surf, they make Athletics checks against waves while the crab moves freely. Reward smart positioning. A PC who climbs onto a high rock has effectively ended the encounter for that crab, since the walk speed is 30 and the climb is none. The crab will menace from below for a round and then look for easier prey.

Time the first reveal to the moment the rogue is checking the tide pool for treasure. A pinch on the hand is the perfect punctuation for a session that started with the bard wanting to look for shells.