Once an organism begins to harness mana into its biology, it starts to develop a biological need for it. If the organism does not have mana flowing through it, it will begin to suffer Mana deficiency. Any organism suffering Mana Deficiency will start to decay, warp, and weaken.
When the body does not get any mana for some extended period of time.
The body of the organism will weaken during the transformation, similar to decaying In Townsfolk, their body and limbs will start deforming, and they'll turn dry and grey. They'll proceed to go mentally insane and lose the ability to talk or even walk correctly. Prolonged effects result in their soul decaying, turning hollow.
Any method of absorbing mana from something will stop further degrading, but it will not reverse the transformations.
Affected organisms have evolved to utilize mana in their biology. One this evolution takes place, all their descendants will suffer from it as well.
Mediation will allow the body to absorb mana from the environment around you, to prevent deficiency
Those who suffer from deficiency for too long become what they call a "Hollow". People see this like a brain-dead walking corpse. Some people may simply put you out of your misery if you've gotten too far gone.