To obtain it, you have to own 2 Mithrill Hammers and then get a desolator recipe from the level 3 recipe shop.
Desolator gives you +50 additional damage and an orb effect which causes armor decrease in enemies that you hit. It also changes projectile art of your attack.
Note: exactness of the information (e.g. damage) can depend on the version of the map
Player2: hmmm.... i think i'll make a desolator..