This is a really good weapon, but hard to get.

1- Acquire an entire set of BRIGHT Plate Armor
2- Acquire an entire set of DARK Will User Robes
3- Have your Bandit Gear ready.

Talk to the Demon Door first with your Bright Plate Armor, then your Dark Will User Robes, then Bandit Gear. It should open, and in a chest in there is the Dollmaster's mace.