Asmodeus

"They do not call it the Asmodean Museum!" said Charles hotly.  His face was flushed and his eyes were protruding a little, which was very impressive for a ghost.  Standing next to him, in a guise only marginally hideous to human perceptions, was Asmodeus.
They used to, he said, his voice seemingly formed from the noises of the cathedral around them.  A footfall became a hard consonant, the flick of a duster across the top of the pews became a sibilant.  It had the odd effect of making it seem like the entire cathedral was speaking Asmodeus's thoughts.  Then Herbert became the curator and decided that the name needed to change.  But didn't the Bard observe that the name of the thing matters not?  That the essence will out?
"That doesn't sound all that much like Taliesin to me," said Charles.  "Not enough words for starters.  And I think he'd probably have said it in Welsh."
Not that... oh never mind.  Asmodeus sighed, which required the doors of the cathedral to swing open and a gust of air to rush in.  Coats fluttered and dresses lifted and an elderly Cardinal who'd led an especially sheltered life fainted.  The choirboys retreated en masse from his prostrate form, leaving it lying on the marble-tiled floors in front of the altar, and two other Cardinals came up to sit him up and revive him. One of them dipped a hand in the font of holy water and offered it to the old man.
"Isn't that sacrilege?" asked Charles.
Hardly, said Asmodeus.  It's tap water most days, though occasionally the cleaning lady empties her bottle of spring water in there if it's looking low.  They only bless it if they remember, usually before a Christening.
"Isn't that sacrilege then?"
Heh, quite possibly. And yet it happens.
Asmodeus looked around and pointed a thumb.  On each hand he had two thumbs, one at each end, and a further five fingers.  Small spikes of bone poked through the skin at each knuckle and there was a leathery, aged quality to the skin, as though it had been tanned without ever being first removed.  He walked in the direction he was pointing, passing invisibly through the cathedral.  Charles followed, noting that Asmodeus's footsteps glowed briefly in the air as he went, wondering if it was a reaction to being on Holy Ground or if it was just an affectation.  He suspected, having spent some time with the daemon now, that it was an affectation.
They left the main body of the cathedral, passing through a robing room where the choirboys had lockers and stowed their stuff while they were officiating in the ceremonies, and then on into a secondary chapel.  The ceiling was high and vaulted and stained glass windows let light in from three of the four walls.  The floor was made of large stone slabs, as were the lower walls, and as Charles looked at them he saw that they were inscribed and realised that they were grave markers.
They buried the wealthy and the powerful here, said Asmodeus.  His voice was much quieter as there was less ambient noise to work with.  An obvious folly, you would think.
"What do you mean?" said Charles.
Asmodeus reached out, his arm stretching and growing, longer and longer until it reached across the room and touched a vertical stone slab.  Light illuminated the inscription on it, growing brighter until it hurt Charles's eyes to look at it.  He turned his head away, having only managed to read the name at the top of the inscription: Lady Sheilagh ap Cwmlleg.  The light faded somewhat, and he turned his head back.  The stone slab had become transparent and behind it, sat at a writing desk with her head held in her hands, was a woman.  She had long, flowing hair that reached to her ankles when she sat and was wearing a ruffled and rouched dress that seemed old-fashioned to Charles's eyes.
Lady Shelagh, said Asmodeus and the woman lifted her head.  Black holes where her eyes should be seemed to suck in the light and her face seemed to sink into shadow.
Lady Shelagh was buried here because she paid a full third of the costs of construction, said Asmodeus.  She didn't nag her husband overmuch, she managed with the one for her entire life, and she entertained various members of the church when they came to see the progress of the work of building the cathedral.  She was considered to be a good woman.
Lady Shelagh hissed at Asmodeus and lowered her head again.  Something rattled on the writing desk and then a bottle of ink fell to the floor and shattered, spattering something red and viscous out into the chapel.
She poisoned each of her children, continued Asmodeus as though nothing had happened.  Two because she had tired of children and didn't wish to be bothered by them; one because she felt it was a sickly child and would not have lived of its own accord anyway, and her first daughter... well, she was poisoned because her mother didn't want the competition.  Did you, Shelagh?
The woman didn't respond, and Charles suspected that the shuddering of her shoulders might be sobs racking her body.
And so, said Asmodeus, Lady Shelagh's presence here, unhallowed and unforgiven, allows me to walk here freely.
"I thought God forgave everything?" said Charles.  "I mean, all you do is repent on your deathbed, right?  And then it doesn't matter how many people you killed, or how you did it, or anything.  You're forgiven, the slate is wiped clean, and off you go to Heaven."
And what do you do there?  Sit around and swap stories about how you murdered your servants for bringing you a drink that was the wrong temperature?
"I... would have thought that would be a sin of pride?"
Asmodues chuckled.  Quite probably.  But you can only be forgiven if you truly repent, saying the words is not enough.  And Lady Shelagh still doesn't regret her actions.
Asmodeus's arm shrank back in on itself, and the stone slab because solid and substantial again.  Charles noted that the spilled ink remained though, a blood-like stain against the wall.
"You showed that to me to make a point," he said.  His words were slow and thoughtful.
Ah yes, said Asmodeus.