Volcanus

Lava Form. A creature or object that touches Volcanus or hits it with a melee attack takes 3d6 fire damage. A creature that ends its turn within 10ft of Volcanus takes 3d6 fire damage. At the end of Volcanus' turn, it ignites any flammable objects within 10ft that aren't being worn or carried. 

Cooled Lava. If Volcanus takes cold damage, it partially cools to stone. Its speed is reduced by 20ft until the end of its next turn and its Lava Form loses function until then. 

Illumination. While Lava Form is active, Volcanus sheds bright light in a 50ft radius. 

Siege Monster. Volcanus deals double damage to objects and structures. 

Multiheaded. Volcanus can't be surprised and it has advantage on saving throws against being knocked unconscious. 

Legendary Resistance (3/day). If Volcanus would fail a saving throw, it can choose to succeed instead. 

Multiattack. Volcanus Roars. It then makes three bite attacks, and one tail attack or a stomp attack. If it is grappling a creature, it can swallow it instead. 

Bite. Melee Weapon Attack: +11 to hit, reach 10ft, three creatures. Hit 3d10+8. If the target is a creature, it is grappled (escape 19). Until the grapple ends, the target is restrained. If Volcanus hits with two attacks using heads next to each other, the escape becomes 25. While Volcanus is grappling, that mouth cannot bite another target. 

Swallow. Volcanus makes a bite attack against the creature he is grappling. If the attack succeeds, that creature takes the bite damage and is swallowed, ending the grapple. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside of Volcanus, and it takes 10d10 fire damage at the start of Volcanus' turn. If VOlcanus takes more than 40 damage, it must a DC20 Constitution saving throw at the end of that damage dealer's turn, or regurgitate all swallowed creatures, which fall prone in a space 10ft of Volcanus. If a creature dies while inside Volcanus, their bodies and all their belongings melt into the magma. 

Stomp. Melee Weapon Attack: +12 to hit, reach 10ft, one creature. Hit 2d6+6 bludgeoning damage and must succeed a DC 19 Dexterity saving throw or be knocked prone. 

Tail. Melee Weapon Attack: +12 to hit, reach 20ft, targets in a 20ft cone behind Volcanus. Targets take 2d6+6 bludgeoning damage and 3d6+4 fire damage, and must succeed a DC 19 Dexterity Saving throw or be knocked back 10ft. 

Roar. Each creature that is within 120ft of Volcanus and aware of it must succeed a DC 16 Wisdom saving throw or become frightened of Volcanus for 1 minute. A creature can repeat this saving throw at the end of each of its turns. Once successful, the target is immune to Roar for the next 24 hours. 

(3/Round) 

Detect. Volcanus Makes a Wisdom (Perception Check)

Tail. Volcanus makes one Tail attack. Volcanus can then move up to half its speed. 

Meteor (Costs 2 Actions). Volcanus makes a Meteor Attack. Volcanus unleashes a 10ft/10ft ball of earth and fire at a target area (ranged 60ft). Each creature within a 30ft radius of the impact must make a DC 16 Dexterity Saving throw. On a failed save, a creature takes 4d6 bludgeoning damage and 4d6 fire damage, and is knocked prone. On a successful save, the target takes half damage and is not knocked prone.