Ink Cloud - Giant Octopus

Monster Manual - 5e, D&D Basic Rules

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.