Miasma is pure poison. Even the slightest contact harms a person, and prolonged contact with it can cause death. Furthermore, it is the substance that is responsible for the appearance of the monsters. It is said that long ago there was no Miasma and no monsters. Although, in Ring of Fates which takes place before the spread of Miasma monsters still exist. This is the same for My Life as a King where the Miasma is now gone but monsters still exist. Sometimes a random Yuke in My Life as a King will question this and say that the "definition" of what people thought monsters were will have to change.