Dark Cathedral
From Tardis Wiki, the free Doctor Who reference
The Dark Cathedral is a portal to the Darksmith Collective's lightless underworld home beneath the surface on the planet Karagula. Few uninvited visitors survived except those striking a deal with the Darksmiths themselves.
Inside the Dark Cathedral can be found a Library of the Darksmith Collective that houses the trading registers and contract details of those 'agreements' undertaken by the Collective. Brother Varlos’s journal on the Eternity Project was stored here in a secret room.
