5e SRD:Giant Octopus
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Large beast, unaligned
Hold Breath. While out of water, the octopus can hold its breath for 1 hour.
Water Breathing. The octopus can breathe only underwater.
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 (Recharges after a Short or Long Rest). 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.