Elemental Nature. The hydra does not need to sleep, eat, or breathe. 

Illumination. The hydra sheds bright light in a 60-foot radius and dim light in an additional 60 feet.

Magma Form. The hydra can move through a space as narrow as 1 foot wide without squeezing. A creature that touches the hydra or hits it with a melee attack while within 5 feet of it takes 11 (2d10) fire damage. In addition, the hydra can enter a hostile creature's space and stop there. The first time it enters a creature's space on a turn, that creature takes 11 (2d10) fire damage and 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.

Multiple Heads. The hydra has twelve heads. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever the hydra takes 30 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies. At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken at least 30 points of cold damage since its last turn. The hydra regains 25 hit points for each head regrown in this way.

If it has more or fewer than twelve heads when it completes a long rest, it returns to having twelve heads.

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Limited Water Susceptibility. For every 50 feet the hydra moves in water, or for every 50 gallons of water splashed on it, it takes 1 cold damage.

Multiattack. The hydra makes as many slam attacks as it has heads, and can then create new heads.

Slam. Melee Weapon Attack: +11 to hit, reach 20 ft., one target. Hit: 11 (1d10 + 6) bludgeoning damage plus 16 (3d10) fire damage. 

Create Head. The hydra can create up to six new heads. It loses 25 hit points for each head it creates in this way. 

Pyroclastic Annihilation. If the hydra has more than 1 head, it can destroy as many of its remaining heads as it wants to create an eruption of magma and volcanic gas that rapidly cools into stone. Each creature within 90 feet of the hydra must make a DC 17 Dexterity saving throw. On a failure, the creature takes 2d10 fire damage plus 2d10 poison damage for each head the hydra destroyed, and is restrained. It takes half as much damage and is not restrained on a successful one. 

A creature restrained in this way takes 22 (4d10) fire damage at the start of each of its turns, and can make a DC 17 Strength saving throw as an action to break out of the cooling magma, ending the effect on itself on a success.