Octopus

An allied octopus is a familiar, a chaos pet, or a bound creature in a wizard's tower aquarium. Find Familiar or a sea-themed warlock's imp produces this framing. The octopus is one PC's responsibility, with a name and a personality.

In a scene the octopus is a scout and a thief. Stealth +6 lets it slip under doors and through grates. Darkvision 30 feet means it sees in the engine room or the flooded crypt without a torch. Swim 30 feet covers distance underwater per round. With a wizard relaying its senses through Find Familiar, the octopus walks the route ahead and reports. It can carry small items: a key, a folded note, a coin.

Combat is not its job. 3 HP and Strength 4 means a single area effect drops it. Keep it in someone's pocket during fights and let it surface for a flavor moment after. If it dies, the controlling PC loses something real.

Have the octopus signal danger by changing color. The PC who handles it learns the patterns over time, and a sudden white flash means the next door is wrong.

A regular octopus is not a combat threat, it is a scene problem. CR 0, 3 HP, AC 12, Strength 4. A single longsword swing erases it. The encounter is what the octopus does before initiative rolls: it slips into a satchel, it wraps around a magic item, it pulls a torch into the water and the room goes dark. Stealth +6 and Darkvision 30 feet let it move around an underwater chamber while the party squints at murk.

It has Tentacles at +4 doing 1 bludgeoning damage and Compression for moving through 1-inch gaps. The real action is Water Breathing (octopus only) and Ink Cloud: once per day, a creature ending its turn within 5 feet triggers the octopus to release ink that fills a 5-foot cube, turning it Heavily Obscured for 1 minute, and the octopus swims away at 30 feet.

Use the octopus for theft, not combat. A successful Tentacles hit on a grabbed creature triggers a Strength check or an item is stolen and pulled underwater. The octopus jets to a crevice with 30 feet of swim and waits. The interesting beat is the chase through a chamber of rocks and cracks. A torch held to a particular fissure will sometimes coax it out. Animal Handling at DC 12 trades food for the item.

If the party tries to fight, it dies in one hit. Don't draw it out. Run multiple octopuses instead on a pearl-diver scene: three to five means every PC is grabbing for a different lost item.

Stage the encounter as a missing-object mystery. The party already lost something to the water. The octopus is the explanation.