Ancient False Hydra

Innate Spellcasting. The false hydra's innate spellcasting ability is Charisma (spell save DC 21). 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 (4/Day). If False Hydra fails a saving throw, it can choose to succeed instead.

Regeneration. The False Hydra regains 20 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 4th 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 21 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.

Enslave (3/Day). The Ancient False Hydra targets one creature it can see within 30 feet of it. The target must succeed on a DC 21 Wisdom saving throw or be magically charmed by the Ancient False Hydra until the Ancient False Hydra dies or until it is on a different plane of existence from the target. The charmed target is under the Ancient False Hydra's control and can't take reactions, and the Ancient False Hydra and the target can communicate telepathically with each other over any distance.

Whenever the charmed target takes damage, the target can repeat the saving throw. On a success, the effect ends. No more than once every 24 hours, the target can also repeat the saving throw when it is at least 1 mile away from the Ancient False Hydra.

Viscous Devour (Mythic trait; Recharges after a Short or Long Rest). If the Ancient False Hydra is reduced to 0 hit points, it doesn't die or fall unconscious. Instead it will devour several of its many followers and regains 297 hit points, and moves up half its movement speed without provoking opportunity attacks.

Multiattack. The False Hydra takes 7 attacks, each with a different head.

Bite. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 19 (2d12 + 6) piercing damage damage plus an extra 18 (4d8) psychic damage. If the target is a Large or smaller creature, it is grappled (escape DC 21). Until this grapple ends, the target is restrained, and the False Hydra can't bite another target with that head.

Swallow. The False Hydra makes one bite attack against a Large 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 28 (8d6) acid damage at the start of each of the False Hydra’s turns.

If the False Hydra takes 45 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.

Song of Forgetfulness. As an action the False hydra can begin its song. The song has a range of 240 ft. and last for 1 minute as if it were concentrating on a spell. Creatures within the 240 ft radius of the song must succeed on a DC 21 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.

Forgetfulness (Recharge 6). The False Hydra targets one creature it can see within 60 feet of it. That creature must succeed on a DC 21 Intelligence saving throw or become stunned for 1 minute. A stunned creature repeats the saving throw at the end of each of its turns, ending the effect on itself on a success. If the target is stunned for the full minute, it forgets everything it sensed, experienced, and learned during the last 5 hours.

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.

If the Ancient False Hydra’s mythic trait is active, it can use the options below as legendary actions for 1 hour after using Viscous Devour.

Swarm. The Ancient False Hydra commands up to 5 of its dominated slaves to move in the way of Ancient False Hydra and use their reactions to attack.

Eat Memories (Costs 2 actions). The False Hydra targets one creature it can see within 5 feet of it. The target must succeed on a DC 21 Wisdom saving throw or take 18 (4d8) psychic damage and become memory drained until it finishes a short or long rest or until it benefits from the greater restoration or heal spell. Constructs, Oozes, Plants, and Undead succeed on the save automatically.

While memory drained, the target must roll a d4 and subtract the number rolled from its ability checks and attack rolls. Each time the target is memory drained beyond the first, the die size increases by one: the d4 becomes a d6, the d6 becomes a d8, and so on until the die becomes a d20, at which point the target becomes unconscious for 1 hour. The effect then ends.

The False Hydra learns all the languages a memory-drained target knows.