Homebrew False Hydra Statblock

False Hydra creature 6
rare ce huge aberration
Perception +17; darkvision, tremorsense (imprecise) 30 feet
Skills Acrobatics +11 , Athletics +15 , Society +13 , Stealth +15
Str +4, Dex +2, Con +5, Int +2, Wis +4, Cha +6
AC 20 (all-around vision); Fort +14, Ref +11, Will +17
HP 120; Immunities blinded, controlled, deafened, death effects, sleep; Resistances mental 5, sonic 5
Speed 5 feet; burrow 20 feet
Wail [[R]] (auditory, mental);
Trigger An adjacent creature hits the False Hydra with a melee attack. Effect All creatures within 5 feet take 4d6 mental damage with a basic DC 24 Will save. Using this ability ends the ongoing effects of Amnesia Song.

Melee [[A]] jaws +13 (agile, reach 20 feet), Damage 2d4+7 piercing plus grab

Abducting Bite [[A]] (attack)
The False Hydra attempts an Athletics check against a grabbed foe's Fortitude DC. On a success, the False Hydra pulls the creature 10 feet closer to it (20 feet on a critical success).

Amnesia Song [[A]] (auditory, concentrate, enchantment, mental, occult);
Frequency Once per round Effect The False Hydra sings an eerie melody which causes creatures to ignore it and forget its existance along with its victims. Each creature that can hear the False Hydra must attampt a DC 22 Will save to resist the effect. The effect lasts for 1 round, but if the False Hydra uses this ability again on subsequent rounds, it extends the duration, causing all affected creatures to reattempt the save. If a creature critically fails a save during the song's duration, further saves each round during the duration cannot get a result above a critical failure.
Critical Success The target is unaffected.
Success The target does not notice the song and has difficulty focusing, treating the False Hydra as concealed.
Failure The target loses all memories of hearing the song and has difficulty directly perceiving the False Hydra, treating it as hidden. If the False Hydra was undetected by the target before this effect, treat this result as a critical failure.
Critical Failure The target loses all memories of directly perceiving the False Hydra and can no longer perceive the False Hydra as long as the song continues. If the target does not have knowledge of the False Hydra outside of their previous perceptions, they lose all memories of the False Hydra. If another situation would convince the target the False Hydra is there (such as seeing an ally attacking the False Hydra), the target can spend an action to reattempt the Will save.

Devour into Song [[AAA]] (death, occult)
The False Hydra can ingest the body of a creature that died within the last hour, regaining 2d10 Hit Points. Afterward, any target which critically fails a save against the False Hydra's Amnesia Song loses all memories of that creature in addition to its normal effects.

Elongate Neck [[A]] to [[AA]] (morph, transmutation)
With the sound of popping bones, the False Hydra stretches and coils its neck in preparation of its next attack. For each action spent to use this ability, the False Hydra doubles the range of its next jaws attack made this round. As long as the False Hydra has enough reach to go around an obstacle, this attack ignores cover and does not need a direct line of effect.

Sneak Attack
The False Hydra deals an additional 2d6 precision damage to flat-footed creatures.

To explain some of my reasoning behind the design.

Amnesia Song is meant to invoke the signature ability of the False Hydra while functioning both within and outside combat. Outside combat, most creatures should fail the Will save within a minute or two, which gets upgraded to a critical failure and causes them to lose their memories. Within combat, it forces PCs to deal with miss chance as their minds struggle to resist the song's effects. Failures on the save no longer upgrade to critical failures, and even a critical failure can be recovered from (at the cost of action economy) as long as the rest of the party is engaged in combat.

Devour into Song is meant to invoke the other part of the False Hydra's signature ability, which somehow never seems to be represented in other statblocks I've seen. The healing is primarily to give temptation to use this in combat and temporarily end the song, as a three action activity leaves no actions left to sustain the song.

Abducting Bite / Elongate Neck Something I see in other statblocks for this creature are giving it normal reach for its size category compared to the original story which seems to emphasize the length of the necks and how it pulls people into discreet locations in order to consume them. So I gave the False Hydra a bit longer than normal reach for its size along with abilities to increase its reach further and actually drag creatures towards it.

Defenses I deliberately gave the False Hydra just below Low AC for its level, as it primarily tries to rely on the miss chance created from the concealed and hidden conditions. High hit points let it survive a bit even if PCs block their hearing to avoid the song, but overall isn't meant to be a creature well-adjusted to a straight up fight.

Offense Continuing with the idea that this isn't a creature well adapted to direct confrontation, I gave the False Hydra low attack and damage for its level. However, it excels it taking advantage of foes who are unaware of it due to its signature song. Both attack and damage improve against anyone who fails a save against its song - since the hidden condition makes the target flat-footed against the False Hydra, lowing their AC and enabling sneak attack to function for extra damage.