False Hydra stats

Common wisdom holds that false hydras come from the ground.  They spontaneously originate as undifferentiated masses of flesh. Potatoes that sprout from no seed.  Supposedly, they germinate in response to lies, and that each falsehood causes a false hydra to swell larger.

Scholars agree, because they have no better idea.  In fact, so much about these abominations boggles the mind that scholars really don't know where to begin.

Paranoia dominates any discussion about it.  Everyone wants to know: Is it here?  Is it in my town?  Is that long, flaccid face watching me through the window even now?

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Notes - I would say just use the stats for a hydra, with the song being an effect that constantly works (no save) unless the PC is immune to being charmed. Maybe once they start looking for the hydra, or they realize that there is a song happening, they might be able to Wisdom save (DC 20) to negate the effect for a short period of time (a minute?), or Counterspell as if the effect were a 7th level spell, again for a short period of time.

If you want the power level of the false hydra to scale up, use the base ability scores of the hydra (MM 190), but the hydra starts at 3d12+15, or 33, HP, and gains another 2d12+10 for each head it has, and it has a proficiency bonus of 1, +1 for each head it has.
I would start it at Small size (2 feet tall), and every time it eats a person it grows a foot in height. At 4 feet, it becomes Medium size, at 8 it becomes Large, at 16 it becomes Huge, and at 32 it becomes Gargantuan. Every ten feet of height, it grows a new head (2nd head at 10 feet, 3rd at 20, etc).

Obviously, feel free to tweak these numbers to better work in the game you're running, and good luck!