Boogie-Hydra

Hold Breath. The hydra can hold its breath indefinitely.

Multiple Heads. The hydra has five 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 142 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 fire damage since its last turn. The hydra regains 10 hit points for each head regrown in this way.

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

Wakeful. While the hydra sleeps, at least one of its heads is awake.

Spell Resistance. The hydra has advantage on saving throws from spells.

Savage Blood. When the Hydra only has one head, it can score a critical hit on a roll of 18-20.

Legendary Actions. The Hydra can make 3 Legendary Actions per turn.

Legendary Resistance (3/Day). If the Hydra fails a saving throw, it can choose to succeed instead.

Multiattack. The hydra makes as many bite attacks as it has heads.

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 30 (4d10 + 10) piercing damage and an additional  30 (3d8) acid damage.

Swallow. When the hydra makes a bite attack, it can choose to swallow its victim.

If the target is a Huge or smaller creature, it must succeed on a DC 26 Dexterity saving throw or be swallowed by the hydra. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the hydra, and it takes 24 (6d8) acid damage at the start of each of the hydra turns.

If the worm takes 60 damage or more on a single turn from a creature inside it, the worm must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the worm. If the worm dies, a swallowed creature is no longer restrained by it and can escape by using 20 feet of movement, exiting prone.

Spit Attack (Recharge 5–6). The Hydra spits an acidic Flem in a 120 feet area. The creature targeted makes a DC 20 Dexterity saving throw, taking 80 (20d8) acid damage and are knocked prone and are stunned for a turn on a failed save, or half as much damage on a successful one and no other effects. Acid fog fills a 10-foot-radius sphere centred on the targeted creature. The fog spreads around corners, and its area is heavily obscured. A creature that ends its turn in the fog takes 9 (3d6) acid damage. A wind of at least 10 miles per hour disperses the fog. 

The Hydra can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.

Bite Attack. The Hydra makes a Bite Attack.

Tail Slap. The Hydra smashes a target within range. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage. On a hit, the target must make a successful (DC 16) Dexterity Save or be knocked Prone