Hildigrim points at the floor near the bookshelf. "There! He could be there!"

Quillary growls and says, “He is definitely here, he countered my spell.” She keeps a passively perceptive eye out for anything moving over the icy terrain towards the exit and escape.

"This urn is full of bone Ash."  Jorin says.

(After Hildi and Quillary speak)

"Show yourself coward!" he shouts.

(Get your hankies, because I'm sure everyone is going to shed tears about the death of Viallis. /s )

Everyone's attention is drawn to the bookshelves, where a man-shaped area of the ash cloud is floating unusually. The fanciest and oldest scroll on the shelf is pulled off and opened.
Before he reads from it, Lord Viallis makes a speech to the party. He is invisible, but you can sense the sneering arrogance on his face.
He may have been cornered (and low on spells), but he seems confident about having a way out.

"You troubling insects! Your surprise attack may have worked today, but Great Gaernoo cannot be denied. I will raise his army and gain rightful rule of these lands. After you are captured by my followers and dragged before me, you will be kept alive for months, wracked by the most painful torture, to pay for your insolence!"

Before anyone is able to stop him, Viallis finishes reading the scroll.
The room begins to brighten as magic energy flares up around Viallis.
But then the energy becomes sparking and erratic. With a scream of pain and denial, Viallis takes 21 force damage.
He only becomes visible for a moment before his body explodes, leaving a darkened crater in front of the bookshelf.

The party is able to avoid being splattered by Viallis parts.
But Hildigrim lets out a gasp of anguish at the books and scrolls on the shelf that will need serious cleaning.

Quillary rises from her viscera-avoiding crouch, clearly annoyed at not getting to smite Vialis. She looks around at the others and the room. “Perhaps now Hildigrim can take the time he desires to read all the words. Jorin of the Bodt, if you would join me, we should make sure the cult is truly driven out. Even a small weed can take root and break stone.” She prepares to stalk the hallways methodically scaring former cultists and sending people home.

Jorin opted to stand upright and cover himself in the flying viscera.

He will happily follow Quillary hunting stragglers.

"That was ... anticlimactic," Hildigrim comments. Volo flutters down to land on the halfling's shoulder. Hildi turns to nod at Quillary as she declares her intentions and heads off with Jorin. He then looks up at Drusk. "I know the answer, but you wouldn't happen to have a bag of holding you've been keeping secret, would you?" He then heads over to the bookcase to take as many books and scrolls as possible.

The day after the party returns to the Road's End, Hildigrim does enough arcane research to figure out what happened to Viallis.
(Put in spoiler only to save space.)

Drusk hands Hildigrim an empty sack to fill up.
"It is only an ordinary bag, so you have to be selective in what you take. I realize this will take some time to inventory what might be useful, but skim over them as quickly as possible. You still have the bookshelves in that earlier office to look through, right?"
(Is Hildigrim skimming, reading intensely, or doing something else?)

For Jorin and Quillary: where do you go?
You could look over the bodies of the dead cultists. There is that cargo-container area in the northern part of the temple. There is Viallis's office that you went through earlier. And there is the lower level, the southern part that wasn't explored. (Or maybe you had something else in mind?)

Quillary intends to start at the furthest end (so probably the unexplored lowest level) and work towards the surface, driving any cultists before her and destroying anything having to do with Vialis’ nascent cult.

I was thinking clearing out the unexplored area as well.  Jorin is equally concerned with smiting hiding cultists and freeing any prisoners that might be in the unexplored areas.

On the way out of the temple, Jorin and Quillary try the door of the unexplored area. (The grey rectangle north of the temple benches for the map in post 5707.)
The door is unlocked. A glowing crystal sphere illuminates this chamber, which has a wooden floor and contains a couch and a tall bookcase filled with books.
The room is uninhabited, so the pair continue north.

After opening the double doors to the entrance hall, they continue east to the staircase down, travel back to the area where you found the prisoners and the cultists practicing magic.
The passageway south is dark, forcing Jorin to draw the moon sword to illuminate your way. The stonework of the walls is more rough and unworked than the passages you've passed through so far.
After going 15' south, it branches to 4 different passages: west, southwest, southeast, and east.

(You are now underneath the temple on the floor up.)

Start with west and if it is uninteresting, work our way around the compass points.

Hildigrim is going to eventually come to the conclusion that he needs more time and will suggest he spend as many hours as it takes to read enough of every book and scroll in the caves to take back with him the ones he wants/needs/should analyze further. He won't sleep unless he becomes exhausted until he's done.

Drusk will be somewhat exasperated, but there is no dissuading Hildigrim when he sees this task that needs to be done.

The books and scrolls contain arcane formulas, alchemical recipes, ritual instructions, and lore about the Outer Planes, half of which is written in Deep Speech. (Deep Speech is the foul language of mind flayers and beholders.) Hildigrim doesn't know the language, but that doesn't deter him from looking over the texts to learn what he can.
(With Hildigrim wearing that magic amulet and his psionic magic training, that explains part of the reason he is reading these texts so intensely.)

Interspersed among the older books are several newer journals written by Lord Viallis.

For the lower level, Jorin and Quillary discovery that the west, east, and southwest passages lead to small rooms with mounds of ash and bone. There are openings in the ceiling that lead to temple above. These are the "cleaning pits" that the "janitor" cultists used to clean up after Lord Viallis’s rituals. (Those 4 "X" areas on the temple map, east and west of the temple pews.)

The path to the southeast is longer, and you've already accounted for all 4 garbage dumps, so it must lead to something else.

Quillary holds a whispered conversation with Jorin to that effect, and then continues down that passageway.

"We will have to tell Hildi about the glowing crystal..." Jorin says, a bit apprehensively, and continues clearing the area with Quillary.

As he is reading, a thought occurs to Hildi, and he asks Drusk, though his eyes never leave his current specimen, "Do you suppose she's gone? That she can finally rest?"

Quote from DirtyDogP >>

As he is reading, a thought occurs to Hildi, and he asks Drusk, though his eyes never leave his current specimen, "Do you suppose she's gone? That she can finally rest?"

Drusk thinks a bit, his fingers tapping a rhythm on the metal urn full of dust and bones. "Ah, you mean the ghost Sarah? She indicated the souls of Lady Maria, Malik, Janus, and Tianna have not moved on since their deaths. All four are trapped in the mortal realm until their physical remains can be buried in hallowed ground. Sarah can't rest until they do. We need to find the bodies now. I hope they aren't in that unholy pile we found downstairs."

Hildigrim goes back to the journals, and skims over the entries since the murders 5 years ago. (Viallis goes into details about how he slaughtered his family as part of a ritual to gain power from Gaernoo, so that journal should go to the Waterdeep guards.) Hildi finds one journal that reveals Viallis placed the cremated remains of his wife and three children in a golden urn that he sometimes uses as a focus for his dark rituals.

Both Drusk and Hildigrim stare at the large metal urn, which (before Jorin partially smashed it) could have held the cremated remains of several people.

Quillary and Jorin find passages that are incomplete, excavations to expand the space below the tower. They also find stairs leading up.

In the room at the top of the stairs are 2 chests. Inside are various personal items and clothes. Some of the items have name tags, such as underwear boxers with the stitching "Sir Vecken of the Broken Horn".

They also find a short corridor that leads to a sliding door panel, same design as the other secret doors you found earlier. Sliding it open, they are back in the southeast corner of that big temple room. They can hear Drusk and Hildigrim still talking in that secret room in the southwest corner.

(You've explored the entire temple and tower. No more fighting for this module. The rest of this should be role-playing: how you want this adventure to conclude, the rooms you want to investigate, who you want to talk to back in Greenfast, etc.)

Quillary conveniently overhears Drusk remind Hildigrim that we need to find the bodies of the dead folks and comes back into their room to also stare forlornly at the urn. She goes over to it and examines the mess. Is it salvageable?

OOC > Anyone have Mending? It's a cantrip, so I don't think I can take a Long Rest and learn it, though it is available to druids.

"Can we not just scoop the ashes into a proper grave?"