The Characters
Sister Rhiannyan, member of the Sisters Repentia who seeks atonement for the galaxy's many sins
Absalom Athanasius, sanctioned Imperial psyker with a talent for telekinesis
Erastus van Saar, ancient and venerable savant who hails from a Hive World of scum and villainy
Sister Lucrezia, a Sister Hospitaller adept at both healing and confronting heretics with a hand flamer
Events
After the execution of Bulagor Thrungg, Inquisitor Sebaton Valker ordered his retinue to investigate the demonic hand that the corrupt nobleman had wielded. Their inquiries brought them to a site called the Haematite Cathedral, a sprawling black edifice festooned with gargoyles. The cathedral appeared to be abandoned; the cathedral was perched atop a cliff overlooking a tumultuous, slate-gray sea. The surrounding area was overgrown and the gardens lay in ruin.
When the retinue entered the cathedral, they immediately noticed that the nave featured a massive cannon. Erastus examined the cannon and found that it could be made functional with only a modicum of repair. Furthermore, the cannon was still loaded. Near the cannon was a grandfather clock that was still keeping accurate time. Next to the clock was an impression in the wall shaped like a cog inset with a human skull.
After Sister Rhiannyan cut through a door that had been painted over with her Eviscerator, the group found a corpse laying on a bed. The body had been stabbed through with a dagger; the dagger's hilt had the initials G.H. inscribed upon it, which led them to believe that the dagger may have belonged to Gustavus Hekate. Upon further examination, they saw that the corpse's skull had three eye sockets--it seemed that they had found the body of a Navigator who had been murdered within the Haematite Cathedral.
Absalom detected the presence of the Warp inside a large, walk-in oven in the Skulleria. A hidden door opened up into a sweating stone staircase leading down into the darkness. The stairs ended in a bronze door; the area smelled strangely of preservative chemicals. The chamber beyond the bronze door was filled with large cages and operating tables crowded with both medical and occult instruments. A complex symbol was inscribed on the floor. More horrifyingly, the back wall was "decorated" with a number of corpses that had been modified and turned into artificial grotesqueries.
Erastus could tell that the markings on the work surfaces in this chamber indicated an interest in alchemy and the pursuit of eternal life. He also noted the symbol of the Ruinous Power known as Tzeentch among the symbology present in the room. Flipping a switch in the chamber caused the wall mounted with deformed corpses to slide open, revealing a hidden part of the room. Oddly, the only thing they spotted in this secret space was a large loose flagstone. When Sister Rhiannyan heaved it aside, a terrible charnel smell filled the room. They could see a multitude of corpses and brown sludge clogging the pit beneath the stone.
Moving the stone also caused a manifestation to occur: a man wearing navy-blue formal attire appeared in their midst. His hands were bleeding and he had a massive wound that tore through his midsection, yet he did not seem to be in pain. In fact, he demanded to know why they had broached his inner sanctum and were disturbing his "work." They were face to face with the Warp specter of Gustavus Hekate! A goading word from Sister Lucrezia infuriated Gustavus, and he launched himself into combat against the group.
He was joined by ambulatory corpses that kept emerging from the pit; Erastus managed to put a few of them down, but ultimately Sister Lucrezia blasted the pit with her hand flamer, nipping that problem in the bud. Gustavus proved to be very difficult to wound. He became immaterial as they pressed their attacks against him, but in the end he too was set ablaze and melted into ash. He left behind a metal object that appeared to be one third of the cog-and-skull that would fit the indentation they had found earlier in their exploration of the Haematite Cathedral.
After a brief rest, the retinue continued to search the cathedral. Erastus took an interest in an antique hunting rifle, so he took that with him. They found an unsent letter from one Vorkas Hekate to the planetary authorities; the letter was meant to alert said authorities to the suspicious conduct of Koronath Hekate. Beneath a glass display dome, they found a letter of warrant authorizing a family known as the Zanatovs to operate as Rogue Traders, which struck them as odd as everything else in the cathedral seemed tied to the Hekate family.
As they pressed further into this wing of the cathedral, they kept encountering banquets and entertainers, though the entertainers were all corpses arranged around a dinner table. A hidden staircase was located; it proceeded in a strange helix pattern up a tower with a mirrored interior. At the top of the tower, they encountered the Warp specter of Nikaea Hekate, a stout woman clad in an ornate purple ball gown and a towering powdered wig. When the group accused Nikaea of Chaos worship, she attacked them. Luckily, she proved easier to deal with than Gustavus--in no small part to Erastus blasting away with the big hunting rifle. When she "died," she left another third of the object meant to fit into the indentation in the nave.
Speaking of the nave, when the retinue returned to it to breach the northern wing of the Haematite Cathdral, they found that the corpses of Gustavus and Nikaea were propped up in its pews.