Tentacles

Melee Weapon Attack: +5 to hit, reach 15 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage. If the target is a creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained, and the octopus can't use its tentacles on another target.

Ink Cloud

A 20-foot-radius cloud of ink extends all around the octopus if it is underwater. The area is heavily obscured for 1 minute, although a significant current can disperse the ink. After releasing the ink, the octopus can use the Dash action as a bonus action.

Skill: Perception

Skill: Stealth

Hold Breath

While out of water, the octopus can hold its breath for 1 hour.

Underwater Camouflage

The octopus has advantage on Dexterity (Stealth) checks made while underwater.

Water Breathing

The octopus can breathe only underwater.