The Demonic Void is a high level greatsword, which is used by Mercenaries. With a very high attack stat, it is recommended to get one as soon as it is possible to be wielded. It is also useful for boss farming, as it does substantial amounts of damage to enemies, even to Levitania bosses.
A Top Quality Demonic Void is the strongest Greatsword in Fantasy Life for those who do not have the downloadable content.
Obtaining the Recipe
Talk to Steele in the Lava Cave upon reaching the rank of Master in the Blacksmith life.