Octopus

An allied octopus is a familiar, a chaos pet, or a bound creature in a wizard's tower aquarium. Find Familiar produces an octopus form in some campaigns, a sea-themed warlock might have one as their imp equivalent, or a coastal sage employs one to retrieve coins from the harbor floor. The framing is small and intimate: this animal is one PC's responsibility, and the party knows it by name.

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

Combat is not its job. With 3 HP and -3 Strength, a single area-of-effect drops it. Keep it in someone's pocket or in a jar of seawater during fights and let it surface as a flavor moment after the dust settles. If it dies, the controlling PC loses something real, and the campaign should treat that as a beat worth pausing on.

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, and Strength 4 means a single longsword swing erases it. The encounter is what the octopus does before the party rolls initiative: 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 ft. let it move around an underwater chamber while the party squints at murky water.

Improvise a tentacle attack from the bare stat line. Use Dexterity (+2 to hit, reach 5 ft., 1 bludgeoning), with a successful hit triggering a DC 10 Strength check or one carried item is pried loose and pulled into the water. That is the right way to use an octopus on the table: it doesn't damage anyone, it steals the wand the wizard was about to cast with. The octopus then jets to a crevice with its 30 ft. swim and waits for the party to leave.

If the party does try to fight it, the right ending is over in one hit. Don't draw it out. The interesting beat is the chase. The octopus has slipped into a 4-inch crack and now holds the bard's spell focus, and the room has six other crevices it could squeeze through next. Druidcraft, Speak with Animals, or a torch held to a particular fissure will sometimes coax it out. A successful Animal Handling at DC 12 trades it food for the item.

If you need a real threat, run a swarm. Three to five octopuses on a pearl-diver scene mean every PC is grabbing for a different lost item while a shape underneath the boat watches and decides who to follow home.

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