You may be looking for the mission of the same name.
The Haunted Cathedral, also called the Old Hammer Cathedral or the Abandoned Cathedral was a great historic monument of the Order of the Hammer. It was situated in the Old Quarter, more specifically in the walled-off section behind the barricades. It was abandoned during The Catastrophe, which originated from the cathedral itself. Since then, the cathedral was swarming with undead monstrosities, which is why it was referred to as the Haunted Cathedral. Its doors were sealed by special Elemental Wards and could be opened only by using all four talismans at once.
The cathedral building itself is an astonishing architectural achievement by the Hammerites. The interior consists of a big hall, an altar, 3 floors plus a cellar, and it is even equipped with elevators. On the outside it stood high amongst all nearby buildings. It is an old Hammer tradition to build their monuments tall. Some of the rooms of the cathedral were used for storing valuables, and some as armories.
The old cathedral had a cloister complex on its backside, which was restricted for any civilians; only members of the order were allowed to enter. The cloister had a wide range of uses from simple living and studying the Builder's tenets to research, invention and craftsmanship. As the order carried out much of its activities in the cathedral complex and there were frequent public services, we can speculate that the place was the headquarters of the Hammer religion for the time before the cataclysm.