Mortaccio is unlocked after defeating 3000 skeletons, and can then be purchased for 500 gold from the character menu. Once unlocked you need to reach level 80 with this character. Unlike other similar achievements the description does not state that it is necessary to have the Randomazzo unlocked, however I waited until I had the Arcana unlocked so that I was able to make builds that regularly got into the 100 plus range by the end of the 30 minutes. His weapon, the Bone, cannot be evolved.