Please check out Goblin Punch's blog entry where they invented the False Hydra, as it is an incredibly entertaining and inspiring read and does this beastie far more justice than I ever could: http://goblinpunch.blogspot.com/2014/09/false-hydra.html

For this statblock, I am assuming that the Hydra is fairly mature and likely has 6-7 heads already, given my players' power level. If you want to scale it down, I'd recommend reducing its HP and damage according to the CR guidelines, as well as giving it less heads. If you have more players, give it more heads!

False Hydra

Gargantuan aberration, neutral evil

Armor Class 16 (natural armor)

16 (natural armor) Hit Points 350 (20d20 + 140)

350 (20d20 + 140) Speed 0ft., Heads 60 ft., burrow 10 ft.

STR DEX CON INT WIS CHA 23 (+6) 14 (+2) 25 (+7) 14 (+2) 18 (+4) 25 (+7)

Saving Throws Wis +9, Cha +12

Wis +9, Cha +12 Skills Deception +12, Stealth +7, Insight +9

Deception +12, Stealth +7, Insight +9 Condition Immunities charmed, frightened, exhaustion

charmed, frightened, exhaustion Senses passive Perception 18, tremorsense 240ft (body), blindsight 30ft. (head)

passive Perception 18, tremorsense 240ft (body), blindsight 30ft. (head) Languages Common, Deep Speech

Common, Deep Speech Challenge 17 (18,000 XP)

Innate Spellcasting. The False Hydra's innate spellcasting ability is Charisma (Spell Save DC 20). It can innately cast the following spells, requiring only verbal components:

1/Day each: compulsion, confusion, dominate monster

Legendary Resistance. (3/Day) When the False Hydra would fail a saving throw, it can choose to succeed instead.

Multiple Heads. The False Hydra has as many Actions on its turn as it has heads, using one Action per head. Each head can concentrate on a spell effect cast by the False Hydra, but that head must use its Action each turn to concentrate on the effect. For every head that it has beyond one, it has an extra reaction that can be used only to make opportunity attacks. While it has more than one head, the False Hydra has advantage on Saving Throws against being Blinded, Deafened, Stunned, and knocked Unconscious. If the False Hydra takes more than 40 damage in a single turn, one of its heads dies. If its last head dies, it dies.

Howl of Oblivion. The False Hydra's heads can, as their action, sing a howling song that causes any who hear it within 5 miles to ignore the Hydra's presence. This extends to any secondary effects of the Hydra's presence, such as the song itself, evidence of the Hydra's presence or even their memories of the people that the Hydra has killed. Children under 13 years of age are able to remember those killed by the Hydra.

Actions

Bite. Melee Weapon Attack. +11 to hit, one target. Hit: 19 (2d12+6) piercing damage. If the target is Large or smaller, the creature is also grappled (escape DC 19). While grappling a target, the head cannot Bite another target. If a Bite attack hits a creature already grappled by the head, the grapple ends and the creature is Swallowed. While Swallowed, the creature is Blinded and Restrained, has total cover from attacks and effects outside the Hydra, and takes 17 (3d10) acid damage at the start of the False Hydra's turn. If the Hydra takes 20 damage or more during a round from creatures inside it, it must succeed on a DC 23 Constitution saving throw or regurgitate all Swallowed creatures, which fall prone within 10 feet of the head that Swallowed them.

Retract Neck. The head is pulled twice its speed back towards the False Hydra's body. A grappled creature must succeed on a DC 19 Strength saving throw or be pulled along with it. This movement can trigger opportunity attacks.

Slam. Melee Weapon Attack. +11 to hit, one target. Hit: 13 (1d12+6) bludgeoning damage.

Mental Revolt (Recharge 6). The Hydra's heads combine to scream their song of mental domination, causing mental strain as its foes try to remember what they're fighting. All creatures within 120 ft. of the Hydra's body must make a DC 20 Wisdom saving throw, taking 34 (6d10) Psychic damage on a failed saving throw or half that on a success. In addition, the False Hydra is invisible to any creature that fails this saving throw until it attacks that creature. Creatures can use their action to try again to succeed on this throw and end the invisibility effect. The Hydra cannot take other actions on a turn during which it uses this ability.

Legendary Actions

The False Hydra can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary actions are regained at the start of each turn.

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

Slam. The False Hydra makes a Slam attack.

Cast Spell (3 Actions). One of the Hydra's heads casts a spell.