Bag of Holding
The journal indicates that mad Lord Gregor and his high priest summoned an osyluth (bone devil) named Sareth’tuel. Sareth’tuel had betrayed Bazgorca, a Lord of Hell, so Bazgorca trapped him in a dark dimension. When he was freed, Sareth’tuel slew his summoners and put their corpses in his former prison. He then set up something in the sewers called the Leviathan that causes the mists. The purpose of the Leviathan to transform the atmosphere of the area into something more suitable for devils.