What is The Boggy Mun?

The Boggy Mun is the entity that personifies the swamp, he is the most powerful of the Old Ones in that area and he is enraged by the draining of his swamp and the loss of his life that he has been poisoning the members of the village with the Swamp Cough, a fatal illness. He uses this power in order to get Briony to tell the townspeople what is going on and save the swamp, but not before also cursing Rose. He is not exactly as villainous as the others but will stop at nothing to get his way.