Hold Breath. While out of water, the octopus can hold its breath for 30 minutes.
Maneuvers. The octopus uses maneuvers (maneuver save Difficulty 11). It has 1 stamina die which is a d8 and regains its stamina die when it finishes a short or long rest. It knows the following maneuvers:
Underwater Camouflage. The octopus has advantage on Stealth checks made while underwater.
Water Breathing. The octopus can only breathe underwater.
Tentacles. Melee Weapon Attack: +2 to hit. Hit: 1 bludgeoning damage.
Ink Cloud (Recharges after a Short or Long Rest). While underwater, the octopus expels an ink cloud in a 1-meter radius. The cloud spreads around corners, and that area is heavily obscured to creatures other than the octopus for 1 minute, although a significant current can disperse the ink. After releasing the ink, the octopus can use the Dash action without using an action.
As an action, make a melee weapon attack and a Brawn check to grapple the creature.
You can add the stamina die to the attack’s damage (add half on a miss by 4 or less) for each additional stamina die expended.
As an action, you can use the Dash action. If you move at least 3 meters straight toward a creature, you can make a melee weapon attack against it.
On a hit, add the stamina die to the attack’s damage.
On a critical hit, add twice as much damage.
On a miss by 4 or less, add half as much damage.
You can increase the damage for each additional stamina die expended.