Agree with Theresa's corrections, and added a couple of possesive corrections.

The Invisible Enemy's Back Cover

Close to Fedora, a terrifying forest shelters beasts. Protectors of their place. Farmers and notables imagine demons, darkness and spells. A strange event will reinforce the Fedoriens' concerns. On the other side of the mountain, the fire reigns.

The invisible enemy will take you on a thrilling quest for truth. Questions. Uncertainties. Discoveries. Halfway between realism and fantasy, embark with soldiers, priests and scholars in a world (that is) neither completely ours, nor quite another's.

Between sciences and magic, what which will you believe?
Between reason and beliefs, what which will you choose?

The Invisible Enemy's Back Cover

Close to Fedora, a terrifying forest shelters beasts. Protectors of their place. Farmers and notables imagine demons, darkness and spells. A strange event will reinforce the Fedoriens concerns. On the other side of the mountain, the fire reigns.

The invisible enemy will take you on a thrilling quest for truth. Questions. Uncertainties. Discoveries. Halfway between realism and fantasy, embark with soldiers, priests and scholars in a world neither completely ours, nor quite another.

Between sciences and magic, what will you believe?
Between reason and beliefs, what will you choose?

Close to Fedora, a terrifying forest shelters beasts who are the protectors of this place. OR

Close to Fedora, a terrifying forest shelters beasts. The beasts are the protectors of this realm.

Between science and magic, which will you believe?

Between reason and belief, which will you choose?

I would like to read more! I like your style. Your writing draws me into the story. Thanks!

The Invisible Enemy's Back Cover

Close to Fedora, a terrifying forest shelters beasts. They are protectors of their place /home. Farmers and notables/ aristocrats imagine demons, darkness and spells. A strange event will reinforce the Fedoriens' concerns. On the other side of the mountain, (the) fire reigns.

The invisible enemy will take you on a thrilling quest for truth. Questions. Uncertainties. Discoveries. Halfway between realism and fantasy, embark with soldiers, priests and scholars into a world neither completely ours, nor quite another/ anyone else's.

Between sciences and magic, what will you believe?
Between reason and beliefs, what will you choose?

Very nice :-)