Volcanic Denizen. Volcanosaur are native to hot springs and volcanic areas. They are unaffected by extreme heat and can swim in lava, magma and boiling water. They are immune to fire damage. 

Thermal Internal. Volcanosaur shed bright light from their vents in a 15 feet radius and dim light for an additional 15 feet. 

Searing Presence. Any creature that starts their turn within 10 feet of a Volcanosaur takes 7 (2d6) fire damage. 

Fiery Soul. If a Volcanosaur is fully submerged in cold water it takes 12 (2d10) cold damage at the start of every turn until it exits the water. 

Multiattack. Volcanosaurs can make a combination of three slam, rock throw or tail attacks, or three magma darts or one Steam Plume attack. 

Slam. Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 18 (1d12 + 5) bludgeoning damage + 8 (2d6) fire damage. 

Rock Throw. Ranged Weapon Attack: +9 to hit, range 30/45 ft., 1 target. Hit: 15 (1d10 + 5) bludgeoning damage.

Tail. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit: 22 (2d10 + 5) bludgeoning damage + 12 (3d6) fire damage. 

Magma Dart. Ranged Weapon Attack: +9 to hit, range 30/45 ft., 1 target. Hit: 13 (3d6) fire damage. 

Steam Plume. The Volcanosaur's heat vents burn a bright orange as a barrage or super heated steam erupts from within. Every creature in a 30 foot cone must make a Dexterity Saving Throw (DC 17) or take 8d10 fire damage and becomes blinded. On a successful save, the creature takes half the damage and is not blinded. 

Vent Expulsion. When a creature hits a Volcanosaur with an attack, it can choose to react by blasting hot steam from its vents, causing the target to take 7 (2d6) fire damage.