One is dropped after killing the TokHaar-Ket Champion in the third room of the tunnels and should be used on the scales to open the door.
The maul is also one of the weapons found on the floor of the fourth room of the tunnels. However, it is not needed to open the door.
Another maul is dropped after defeating the enemies in the sixth room. Once again, it should be used on the scales to open the door.
A maul is used once again seventh room of the tunnels. It should be placed on a pedestal marked "Ket".