Innate Spellcasting. The false hydra's innate spellcasting ability is Charisma (spell save DC 15). It can innately cast the following spells, requiring no components:

At will: detect thoughts

1/day: dominate monster

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.

Sunlight Sensitivity. While in sunlight, the False Hydra has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Three-Headed. The False Hydra has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious.

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

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.

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.

Legendary Resistance (2/Day). If False Hydra fails a saving throw, it can choose to succeed instead.

Multiattack. The False Hydra makes 3 bite attacks, one with each head.

Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 12 (2d8 + 3) piercing damage damage plus an extra 9 (2d8) psychic damage if the target is a creature. 

Song of Forgetfulness. As an action the False hydra can begin its song. The song has a range of 120 ft. and last for 1 minute as if it were concentrating on a spell. Creatures within the 120 ft radius of the song must succeed on a DC 15 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.