Goblin Punch's False Hydra

[GuesssWho]

As a fan of the Goblin Punch blog, I decided to try statting up the infamous False Hydra. No idea what it's CR would be, but it's really more something to work around than fight anyway.

Huge monstrosity, unaligned
Armor Class 16
Hit Points 172 (15d12+75)
Speed 30 ft. (active phase only)
STR 15 (+2)
DEX 10 (0)
CON 20 (+5)
INT 15 (+2)
WIS 10 (0)
CHA 16 (+3)
Proficiency Bonus +4
Skills Perception +6
Senses darkvision 60 ft., passive Perception 16
Breathless. The false hydra in sessile phase doesn't breathe.
Multiple Heads. The false hydra has one to seven heads depending on age--six or less is a sessile hydra, seven is active. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious. Whenever the hydra takes 25 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies.
Reactive Heads. For each head the false hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.
Wakeful. Only one head sleeps at a time.
Mindsong. As long as more than half the heads can vocalize, a sessile false hydra creates an invisibility effect. Treat as the spell except that immunity to mental effects negates it, as does Silence or deafness. See Invisibility does not affect this ability, and True Seeing merely gives a strong sensation of having forgotten something very important. While invisible, the false hydra is completely unmemorable. An active false hydra is always visible and instead creates a suggestion effect.
Bite Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage. The false hydra makes as many bite attacks as it has heads. Any individual killed by a false hydra is instantly forgotten by everyone who knew them, resulting in much confusion and an uptick in mental instability in the area where it grows.