Magma Octopus

Lava Bound. The magma octopus can exist outside of lava for up to 1 hour each day. If it remains outside of lava for 1 minute or longer, its skin begins to dry and harden like rapidly-cooled magma. While its skin is hardened, it has a walking speed of 20 feet, an Armor Class of 18, and vulnerability to cold damage. If it remains outside of lava for 1 hour or longer, it becomes petrified, transforming into basalt. Its body returns to normal if it is submerged in lava for 1 round.

Lava Swimmer. While in lava, the magma octopus has a swimming speed of 60 feet and can move through the lava as if it were water.

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.