Spellcasting.

The False Hydra is a 8th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 20, +13 to hit with spell attacks). The False Hydra has following spells prepared:
(at will): Dissonant Whispers
(5 per day): Hold Person

(2 per day): Confusion

Abilities

Reactive Heads: For each head the false hydra has beyond one, it gets an extra reaction that can be used only for Opportunity Attacks.

Multiple Heads: The false hydra has a head for each party member +1 (minimum 5). While it has more than 1 head, it has advantage on Saving Throws against being Blinded, Deafened, or Stunned.

If the false hydra takes more than 50 damage in a single turn, or a critical hit lands on the hydra, it loses one of its heads.

At the end of each of its turns, the false hydra regains heads equal to the number it lost since its last turn plus 1. For every instance of fire damage the false hydra took since the heads were lost, subtract 2 from the number gained back. For each head generated this way, the hydra gains 20 hit points.

Hypnotic Song: While at least one of the false hydra's heads is singing, all creatures of the hydras choice within 1 mile must make a DC 16 Wisdom saving throw. On a success, the target takes 1d6 psychic damage. On a failure, the target is charmed by the false hydra, and unaware of its existence. At the start of each of its turns, the target can make a DC 19 Wisdom saving throw to end this condition. If the target can see an ally that is not currently affected by this condition, it has advantage on its save.

Unrelenting Hunger: At the start of each of its turns, the false hydra regains 10 hit points for each creature that it currently has swallowed. For every 40 hit points it regains in this manner, it grows a new head.

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

[title]Spellcasting.[/title]
The False Hydra is a 8th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 20, [+13] to hit with spell attacks). The False Hydra has following spells prepared:
(at will): [spell]Dissonant Whispers[/spell]
(5 per day): [spell]Hold Person[/spell]
(2 per day): [spell]Confusion[/spell]

[title]Abilities[/title]

[b]Reactive Heads:[/b] For each head the false hydra has beyond one, it gets an extra reaction that can be used only for Opportunity Attacks.

[b]Multiple Heads:[/b] The false hydra has a head for each party member +1 (minimum 5). While it has more than 1 head, it has advantage on Saving Throws against being Blinded, Deafened, or Stunned.

If the false hydra takes more than 50 damage in a single turn, or a critical hit lands on the hydra, it loses one of its heads.
At the end of each of its turns, the false hydra regains heads equal to the number it lost since its last turn plus 1. For every instance of fire damage the false hydra took since the heads were lost, subtract 2 from the number gained back. For each head generated this way, the hydra gains 20 hit points.

[b]Hypnotic Song:[/b] While at least one of the false hydra's heads is singing, all creatures of the hydras choice within 1 mile must make a DC 16 Wisdom saving throw. On a success, the target takes [1d6] psychic damage. On a failure, the target is charmed by the false hydra, and unaware of its existence. At the start of each of its turns, the target can make a DC 19 Wisdom saving throw to end this condition. If the target can see an ally that is not currently affected by this condition, it has advantage on its save.

[b]Unrelenting Hunger:[/b] At the start of each of its turns, the false hydra regains 10 hit points for each creature that it currently has swallowed. For every 40 hit points it regains in this manner, it grows a new head.

[b]Legendary Resistance (3/Day):[/b] If the false hydra fails a saving throw, it can choose to succeed instead.

Actions

Sing. The false hydra can start singing by using one attack from its multiattack action. Only one needs to be singing for the song's effects to take place. There is no benefit to multiple heads singing. While a head is singing, it does not count toward the number of heads used in its multiattack action.

Multiattack: The false hydra makes as many bite as it has heads (that are not singing and do not have a creature grappled), and can take the engulf action against one creature it is grappling.

Bite: Melee Weapon Attack: +15 to hit, reach 40 ft., 1 target. Hit: 19 (2d10+8) piercing damage. If the target is a large or smaller creature, it must succeed on a DC 18 Dexterity saving throw or be grappled (escape DC 18). A head that has a creature grappled in this way cannot take the sing action, and cannot take the bite action against any other creature.

Engulf: One large or smaller creature that is grappled by the false hydra must succeed on a DC 18 Dexterity saving throw or be swallowed. A swallowed creature is Blinded and Restrained, it has total cover against attacks and other effects outside the hydra, and it takes 11 (2d10) acid damage at the start of each of the hydra's turns.
If the hydra takes 25 damage or more on a single turn from a creature inside it, the hydra must succeed on a DC 21 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 hydra. If the false hydra dies, a swallowed creature is no longer Restrained by it and can escape from the corpse by using 20 feet of Movement, exiting prone.

[b]Sing.[/b] The false hydra can start singing by using one attack from its multiattack action. Only one needs to be singing for the song's effects to take place. There is no benefit to multiple heads singing. While a head is singing, it does not count toward the number of heads used in its multiattack action.

[b]Multiattack:[/b] The false hydra makes as many bite as it has heads (that are not singing and do not have a creature grappled), and can take the engulf action against one creature it is grappling.

[b]Bite:[/b] Melee Weapon Attack: [+15] to hit, reach 40 ft., 1 target. Hit: 19 ([2d10+8]) piercing damage. If the target is a large or smaller creature, it must succeed on a DC 18 Dexterity saving throw or be grappled (escape DC 18). A head that has a creature grappled in this way cannot take the sing action, and cannot take the bite action against any other creature.

[b]Engulf:[/b] One large or smaller creature that is grappled by the false hydra must succeed on a DC 18 Dexterity saving throw or be swallowed. A swallowed creature is Blinded and Restrained, it has total cover against attacks and other effects outside the hydra, and it takes 11 ([2d10]) acid damage at the start of each of the hydra's turns.
If the hydra takes 25 damage or more on a single turn from a creature inside it, the hydra must succeed on a DC 21 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 hydra. If the false hydra dies, a swallowed creature is no longer Restrained by it and can escape from the corpse by using 20 feet of Movement, exiting prone.