Innate Spellcasting. The false hydra's innate spellcasting ability is Charisma (spell save DC 17). It can innately cast the following spells, requiring no components:
At will: detect thoughts
3/day: dominate monster

Magic Weapons. The False Hydra’s attacks are magical.

Siege Monster. The False Hydra deals double damage to objects and structures.

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

Regeneration. The False Hydra regains 15 hit points at the start of its turn. If the False Hydra takes fire or radiant damage, this trait doesn’t function at the start of the False Hydra’s next turn. The False Hydra’s body is destroyed only if it starts its turn with 0 hit points and doesn’t regenerate.

Limited Magic Immunity. The False Hydra can’t be affected or detected by spells of 1st level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.

 Mental Fortitude. The False Hydra has advantage on saving throws against being charmed or frightened, and magic can’t put the False Hydra to sleep.

Devil’s Sight. Magical darkness doesn’t impede the False Hydra’s darkvision.

Unusual Nature. The False Hydra doesn’t require air or sleep.

Boneless. The False Hydra can move through and occupy a space as narrow as 1 foot wide without squeezing.

Detect Sentience. The False Hydra can sense the presence and location of any creature within 300 feet of it that has an Intelligence of 3 or higher, regardless of interposing barriers, unless the creature is protected by a mind blank spell.

Fear Aura. Any beast or humanoid that starts its turn within 10 feet of the False Hydra must succeed on a DC 17 Wisdom saving throw or be frightened until the start of the creature’s next turn.

Aversion of Fire. If the False Hydra  takes fire damage, it has disadvantage on attack rolls and ability checks until the end of its next turn.

Immutable Form. The False Hydra  is immune to any spell or effect that would alter its form.

Rubbery Skin. Whenever the False Hydra  is subjected to lightning damage, it takes no damage and instead regains a number of hit points equal to the lightning damage dealt.

Nondetection. The False Hydra  can’t be targeted or detected by any divination magic or perceived through magical scrying sensors.

Multi-Headed. The False Hydra has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious. The False Hydra also can’t be surprised.

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

Multiattack. The False Hydra takes five bite attacks, each attack must be with a different head.

Bite. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 14 (2d10 + 3) piercing damage damage plus an extra 13 (3d8) psychic damage. If the target is a Medium or smaller creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained, and the False Hydra can't bite another target with that head.

Song of Forgetfulness. As an action the False hydra can begin its song. The song has a range of 180 ft. and last for 1 minute as if it were concentrating on a spell. Creatures within the 180 ft radius of the song must succeed on a DC 17 Wisdom saving throw or be under the effects of the False Hydra's charm. Charmed creatures are not able to perceive the False Hydra as if they forgot it was there. The effected creatures can attempt the save again at the end of its turn and if a creature has succeeded the save against the song it is immune to its effects for the next 24 hours. The False Hydra cannot attack while singing.

Swallow. The False Hydra makes one bite attack against a Medium or smaller creature it is grappling. If the attack hits, the target takes the bite’s damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the False Hydra, and it takes 21 (6d6) acid damage at the start of each of the False Hydra’s turns.

If the False Hydra takes 30 damage or more on a single turn from a creature inside it, the False Hydra 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 False Hydra. If the False Hydra dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

The False 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 False Hydra regains spent legendary actions at the start of its turn.

Attack. The False Hydra makes one bite attack.

Move. The False Hydra moves up to half its speed.

Detect. The False Hydra makes a Wisdom (Perception) check.