This weapon can only be obtained by killing the Mallorn Ent boss, or getting it from creative mode. When slain, this boss has a 50/50 chance of dropping the mace. Because you must have a negative Fangorn alignment to summon and slay the Mallorn Ent boss, it's impossible for most good players to obtain this weapon without killing another player or having them give one to you. The weapon is therefore difficult to obtain.
This weapon can be repaired on an anvil by combining it with Mallorn sticks.