There was once 12 kingdoms in the ancient days until the kingdom of Eo was devoured by the very forests it called home. The people of Eo have been turned into beasts wandering the woods and preying upon all who enter the forest.
That's how the legend goes anyways. The people of Eo where actually infected by a parasitic organism much like a tick except this tick sustains itself by entering its host and slowly taking over its nervous system and brain which it then uses to sustain its host and thus sustain itself.
What I'm wondering is how the tick would actually take over its host's brain. Something like a direct "plug" into the cerebral cortex, connecting to the top of the spine, etc.
Note: the tick itself is reminiscent of a crab in physical build minus the big claw but is about the size of a Deer Tick, which limits how it gets to the brain to ear ducts, wiggling past the eye balls, or sneaking in through the sinuses.
The Brain Tick causes those effected by it to have a stagger in their walk and slurring of words as their body tries to resist the tick taking over. The tick would also force their host to eat about anything they can physically digest. Eo are regular humans.