Unlocked through a dlc code.
Obtained after defeating Ghorn.
Whether intentional or not, the mace has the status of a possessed weapon. As it only has stats for damage this allows for players to upgrade it with up to four additional attributes.
Also there appears to be a glitch, where if you run the crucible after defeating Ghorn via quest, and don't currently have Mace Maximus in your inventory, it will drop again after defeating him in wave 24. (PC Only)
|“|| A weapon of incredible power, forgotten to the eons. Formerly brandished by an ancient lich warlord whose reign of conquest faded into dust. Locked deep in the tombs below the Dead Plains, thought never to resurface. The only remnant of his tale is inscribed in runes on the weapon's blades.|
— In-Game Description, Darksiders II