Water Breathing. The octopus can breathe only underwater. It can hold its breath for 1 hour outside water.
Actions
Tentacles. Melee Attack Roll: +5, reach 10 ft. Hit: 10 (2d6 + 3) Bludgeoning damage. If the target is a Medium or smaller creature, it has the Grappled condition (escape DC 13) from all eight tentacles. While Grappled, the target has the Restrained condition.
Reactions
Ink Cloud (1/Day). Trigger: The octopus takes damage while underwater. Response: The octopus releases ink that fills a 10-foot Cube centered on itself, and the octopus moves up to its Swim Speed. The Cube is Heavily Obscured for 1 minute or until a strong current or similar effect disperses the ink.
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.