False Hydra

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?
There are false alarms; criminals and deserters have pleaded that they were merely under the influence of the false hydra, or that they were merely trying to escape it's influence. And sometimes that was the truth.
There are ghost towns in the Grey Waste. Victims of false hydras. People do not revisit those sites, out of fear of vengeful ghosts. And perhaps the false hydra is still there, the black rot at the center of the bone. And how would you know?
In gentler lands you will find skeptics. These erudite scholars will stroke their chin and calmly tell you that there is no such thing as a false hydra. It is some confabulation. Villages seized by some infectious insanity, or perhaps some subtle demon.
But they are wrong.

Infiltration

The false hydra enters a town through a humble enough method. Fattened on worms, it has been growing upwards these last few days (weeks? years?), but has only now broken through the soil. It emerges in a basement, from behind the jars of fruit preserve. Or pushes its face up through a broken cobblestone. And then it begins to sing.
While it sings, it is ignored. It just creates gaps in your attention and then slips through them. It is subtler than invisibility, and more reliable.
At this point, the false hydra is only a torso - presumably about the same size as a man's - buried somewhere in the ground. The neck grows up, up until the head emerges from the ground. The head is only the size of a man's head at this point. It resembles a man's head, too, but white, hairless, and with thick deformities of the brow and lips. The eyes are wet holes.
But of course, none of this is noticed. While it sings, the hydra exists in our blind spot.

Growth

The hydra eats people, of course. To eat someone, it must usually stop singing, which endangers the hydra somewhat, since it can now be noticed. To make this task easier, the hydra usually drags the unfortunate victim a short distance underground, into a basement, sewer, or small chamber that it has excavated, and devours them there.
A man is walking along a deserted street. Suddenly he realizes that the silence is more profound, as if a loud noise had just ceased. There is a rattle as a sewer grate slides over rough stone. In that darkness, a fleshy face, leering with undisguised hunger. It lunges forward on a thick neck that slides out of the darkness like a sheath, one foot, three feet, six feet long. And then it bites him on the arm and drags him down that narrow gap, yanking and twisting to fit the man's body through that too-small space. And when the sounds of eating have ceased, the song resumes.
The man has family and friends who will notice his absence, but the song of the hydra massages their mind, smoothing the wrinkles on their brain. The hydra has eaten the man, who is now known to the hydra. The song erases the memories from their soft heads. They will not notice his absence, nor remember him.
And in this way, the hydra grows. The body fattens, swells, its necks stretch long... longer. And with it, its influence.

Dissonance

The false hydra's song hides the memories of the devoured victims in the same way that it hides the false hydra, but this is not a perfect system.
Wives will wonder why there are men's clothes in her closet. People will notice that no one has lit the street lanterns these last few nights. Churches suddenly find themselves without a bell ringer.
By and large, these gaps close themselves up. The wife will forget about the clothes as soon as she stops looking at them. Or she will conveniently remember how her brother left them there the last time he visited. Or she will, on some level, recognize the wrongness implicit in the clothes, and throw them away one moonless night. She will confabulate a narrative to explain and correct these disparities, powerfully and constantly.
But part of her mind is cognizant of the disturbance. That part of her mind is distrusted, and sealed away. But that primordial cluster of neurons still fires. A syphilitic madman who has been locked in the attic by his family, but whose mutters can sometimes be heard during the lulls in the dinner party downstairs.
This creates pressure. In the early stages, this feels like paranoia, especially the sense that someone is watching you (and the hydra is watching you, pressing its moony face up against the window and fogging up the glass). More severe symptoms develop. Reminiscing becomes a stressful and uneasy experience, and so is avoided. Distortions of memory. The confabulations pile up, identities become muddled. Friend's faces seem subtly deformed.
Human brains were not meant to bear this weight. Mundane insanities sprout like mushrooms. Nervous disorders. Psychotic breaks.

Proliferation

As the false hydra matures, it grows more heads. The process accelerates exponentially. More blood on the cobblestones. More incongruities festering in heads like gangrenous limbs. The false hydra gets careless. With every meal, it becomes more powerful, more able to smother mankind. It doesn't need to be careful anymore.
The heads stretch up higher. Their gracile necks sway above the rooftops. Their heads have grown feral. The skull bulges with masses of bone. The lower jaw juts out, low-slung, like a dagger in a fist. Soon, it will finish devouring this city.
But darling, my darling, there isn't enough blood in all the world to slake its thirst.
When a false hydra is mature (some texts localize this event to the day when it has grown seven heads) it begins to sing a new song. This song mentally enslaves everyone within hearing range.
The false hydra orders its servants to exhume its body, now grown swollen and fat. And while they dig, it eats. And then the false hydra orders that it should be transported to a new city, where there is new flesh to be eaten. It will be borne there atop the backs of its slaves, grateful legs staggering under its cold tonnage. When it gets too large to carry they will lash it with chains and drag it behind them like a wailing, blubbery siege engine. (Which it is.)
Of course, this is unsustainable. As soon as a mind-slave is outside the range of the false hydra's voice, they'll flee. Unless it raids other food stores, it will starve. It cannot farm or hunt sufficient food without spreading its servants across an unacceptably broad area.
The uncommon adulthood of false hydras is marked by desperate aggression. An animal convulsing as it dies, crushing people and cities under its hungry bulk. It usually heads for the largest cities, seeking the largest food source. Sometimes it succeeds long enough to grow larger and move on to the next city. A tour of death.
The "traditional" tactic is to set fire to the granaries and evacuate the city. The false hydra will starve to death in a few weeks, while everyone visits their relatives in the countryside. The false hydra's movements are tracked by scouts on horseback, who watch the abomination from the horizons and communicate by flags. Many of them choose to mutilate their own ear canals, in order to deafen themselves.
These tactics failed spectacularly in the summer of 882 TFM, when there were multiple false hydras colluding with each other. (The exact number is still disputed.)
A more pressing problem is bandits, preying on families traveling alone with all their wealth. Looters also linger in the cities after the evacuation order has been given, and many eventually fall victim to the false hydra, and allow it to grow larger. Assassinations and power struggles are also common, as different parties use the chaos to seize an advantage.
And lastly, a military presence must ensure that no mercenary company, slavelord, or evil wizard is allowed to open up lines of communication with the false hydra (using messengers). Those avenues of exploitation have allowed some absolutely horrific tragedies in the past. If not destroyed when young, the cancer must be isolated until it is forced to eat itself.