Multiattack

The magma octopus makes four attacks with its tentacles.

Tentacle

Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 8 (1d6 + 5) bludgeoning damage plus 2 (1d4) fire damage. If the target is a creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained, and it takes 2 (1d4) fire damage at the start of each of its turns. The magma octopus can grapple up to two targets at a time.

Magma Blast (Recharge 6)

The magma octopus sprays magma in a 30-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw, taking 28 (8d6) fire damage on a failed save, or half as much damage on a successful one. If a creature fails the saving throw by 5 or more, it catches fire. Until someone takes an action to douse the fire, the creature takes 5 (1d10) fire damage at the start of each of its turns.