I turn to this illustrious group to give me ideas for what sort of nasty things would be found in an infernal cult's mountain temple. Your thoughts are greatly appreciated.
Is this an obvious Infernal Temple , or does the Cult appear to be something more benign?
Any details on what kind of Infernalists they are , Corrupters , Deceivers or Destroyers?
Is it in an Infernal Regio?
The most obvious thing to tempt magi is books.
Pages 86 , 87 in RoP:TI , have things like Infernal Heirloom , Tainted Treasure , and Corrupted (Abilities , Arts & Spells).
Books teaching any Ability , Art or Spell could be available.
Make the Level & Quality higher than those readily available in your Saga ,
but not so high that players get suspicious.
Use names of Authors from Antiquity (but not original works penned by them) ,
or for Hermetic subjects use House Ex Miscellanea authors.
(possibly integrating non-Hermetic work)
These can be entirely false works using real names.
Unless a player does research at the Domus Magna (Cad Gadu or Durenmar) he will be unable to determine this.
None of these works should detect as Infernal if penned by mortal scribes at the behest of a demon.
(time is not a factor here , some of these works could be hundreds of years old)
A storage box , or cupboard (armarius) that magically prevents texts from aging.
(in fact , over time , the scribe demon in the item subtly rewrites texts , Tainting the knowledge therein)
If you want some specific items designed for your game ,
give us some guidelines on power levels.
The Infernal Heirloom has the equivalent of a Level 25 Hermetic spell , once per day.
Some statues of Cthonic Deities may be appropriate.
These are attuned to the Infernal , but are not demonically tainted.
Along with various votive tablets and fragments of ancient texts ,
you could start the players on a search for some secret of Ancient Magic.
These items were not used by the Cult , but merely stored away , being of no immediate use to them.
Covenants (page 91)
mentions Mercurian Rituals carved into 38 golden tablets , which have been lost.
Copies of what appear to be Mercurian Rituals (not on gold tablets) , currently unknown to the Order , could be found.
Again , clues to a cache of 01 or 02 of these tablets , might be a useful story hook.
Thanks for your response. I guess I should clarify:
(Serf's parma.)
I'm going to run a small travelling side story involving one of the Mentem Vis sources listed in "Covenants." I can't remember the name exactly, but the source is the severed head of a monk who'd been kidnapped by diabolists. The head sheds mentem vis tears. I'm adjusting the original material a bit. The characters are going to get enlisted by a monastery to find their abducted monk, go on a bit of a hunt, and eventually come to the blasted out remains of diabolic mountain temple. The cultists have all been killed, and I need some details of what nasty things the characters will find there. I'm not a particularly dark person, so I always have trouble coming up with the dark and insidious details. Flavor for the temple's description.
I'm running this because I'm very curious if the characters will keep the head as a source of Vis, or give it a church burial. The players are still very weak, and don't even have a covenant yet....
If they choose to keep it, the head will eventually become a tool of the infernal realm to get it's hooks into the characters.
.... that all being said, the characters are travelling through a very remote and poor section of the alps. The cult itself is little better than a bunch of shepherds, led by a twisted scholar. So "Temple" might be the wrong word. Mountain hovel might be better.
So I'm just looking for little details to spruce up the description. Bones of animals in interesting shapes. A little cage where the cultists had kept a child for sacrifice. There'll be a book, containing a few rituals. A book on occult lore. Some herbam and animal vis, some corrupted corpus vis. Maybe some poor lab equipment like dirty vials containing nasty little ingredients.
Probably some silver that has a "greed" curse put upon it?
Individual Runes carved from human bone is always good.
Especially if the Runes were carved from the bones of unbaptized children.
Have a child in the cage (still alive , though probably starved) ,
who may have witnessed companions being dismembered alive to carve their bones into Runic shapes.
Ancient Magics has runic alphabets on page 142 & 143.
The Runes (as a set) contain Vis that may be extracted.
(your choice as to types , but Intellego would be part of it)
This could be a Divination Tool (non-Hermetic) , but as with all Tainted things only brings about Ruin and Destruction.
Alternatively , Demonic sigils have been carved into them.
(studying these sigils will give a bonus to Infernal Lore)
If you want an encounter , have a Corrupted Beast (page 77 , RoP:TI) lairing in a nearby cave.
Maybe a flock of carnivorous sheep that had been tended by the shepherds.
If fed unbaptized children , the wool from the flock may be harvested
to provide a number of pawns of corrupt Corpus vis equal to 3x (or more) the number of children they have eaten.
(this can be done once per year)
Unbaptized children being scarce , the Vis available is higher.
I've run a couple of stories in which the players have had to sift through diabolists' shrines. For flavor, just think of a few pointlessly wicked things diabolists might do and describe evidence of them. Such evidence need not be supernatural; it can even be downright tawdry.
-A pail filled with flies' wings.
-A notebook containing a detailed list of "lies I've told today," or perhaps just a scrap of parchment on which the same lie has been written over and over again.
-A mask with a serpent's tongue.
-Perhaps there's a bruised, half-starved dog chained out back. There's nothing special about it; its diabolist owner just liked to kick dogs.
-A breviary or devotional work of some kind, thoroughly defaced by blasphemous grafitti.
-etc.
I hope that you find some of this helpful, and do tell us how the adventure turns out.
But THIS is fantastically horrible, and exactly the type of suggestion I was looking for.
Thanks so much.
As per Black Death:
Sheep Enthrails prepared as an offering for the demon. Of course if you go with the use of human bone I am sure that the enthrails of the children (extracted under suitably horrific forms) would do just as well or in fact much better.
My own ideas :
Slaves whom have been forced to partake in the dark rituals (tarnishing their souls and preparing them for an eternity in hell). This might yield some interesting companions or grogs. (Same thing can be used for local clergy that has... ahhh... dissapeared during the cultists run.)
A holy relic thoroughly debased in any way the diabolists could possibly concieve of, most of them rather crude.
Various blackmail material on local nobles, perhaps them having signed contracts in blood with either the cult or whatever minor/major demons they have managed to call up?
Magical item that could be very useful... but is very obviously made from human remains and tainted vis. Perhaps a staff containing a spell or two... but made out of a human spine fused together with heat perhaps with the skull still moving as if writhin in pain each time used.
Jars containing something to cath the souls of the victims for Vis-extraction. (QUITE obviously should give tainted Vis this one.)
... a lot of keys to power and temptations for the players.
First off, my apologies to Ravenscroft for not quite understanding how to use QUOTE feature..... I added my own comments into his quote area by mistake.....
I just realized I left my RoP:Infernal book at work. Can someone tell me the formula for changing the tainted vis into regular vis? (and what is the name of the least-evil form of tainted vis. Is it "sordida?")
Thanks,
eric
ps. I'm enjoying the suggestions. Thanks to all!
Depending what you want for your players , you could have wood samples , or a few growing trees as listed on page 33 of GotF.
This would be for the Shape & Material Bonus.
Elder is +04 Malicious Magic , Ash is +02 Harm People , Yew is +02 Corpses.
Page 33 also says , that if an entire tree is enchanted , any such bonus is tripled.
(normally the bonus is capped by your score in Magic Theory)
You could have the Bonus capped at (MT + Infernal Lore) or some other Tainted Ability.
Have some casting tools (a wand or staff) that provide a bonus when used with a Method & Power ,
or usable by a Verditius mage , but have constant effect for Perdo Corpus , or similar damaging spells.
(the bonus is only added to a single Tech + Form)
The bonus for Human Bone & Human skull is on page 110 in the core rules.
(+04 destroy the human body)
RoP:TI has Shape & Material bonus items on page 123.
Candle made of Goat Fat : +03 summon demons.
Candle , black : +02 summon demons.
Sulphur : +04 demons.
So Black candles , made of goat fat , impregnated with sulphur might be found.
(treat as a lesser enchanted device , but made by non-Hermetic means)
If this group had been waylaying travellers ,
they could have disposed of the bodies by composting them to fertilize any plants or trees.
Feeding them to the sheep and burning the dried dung in a ritual brazier.
Vis Infesta on page 18 , is the weakest.
Vis Sordida is next and listed as the most common Infernal vis encountered by Hermetic magi.
Vis Prava being the most vile.
I haven't found a section on converting Tainted Vis into regular , as yet.
Are you sure that this can be done?
Page 18 & 19 list the effects of using Infernal vis , or Enchanting with it ,
but not "Cleansing" it.
No need to apologise about any quotey mishaps. 
Cleanse the Verminous Vis, ROP:TI p. 122.
Guess i failed my spot the obvious check. 
I was looking for a Lab Technique , not a spell.
The spell does say :
Stealing an idea from Xipe Totec ,
the Cult leader has a Ceremonial Casting robe made of human skin.
If examined by the players , it will become apparent that it is made from the skins of a number of children ,
not a single adult human.
Flaying tools used in the process for removing skin can be found.
Due to players having seen a fair number of horror movies ,
it may be counterproductive to have too many obvious trappings.
Any human remains found by the players should be taken for a Christian burial.
Merely burning or burying them without doing so could leave the area haunted or cursed.
Being caught with human remains in your possesion by suspicious locals is also not a good idea.
The cursed silver is a good idea.
Make it the traditional 30 pieces (not the originals) with a Curse of Betrayal.
A wife will commit adultery , a trusted associate bankrupts you in a business deal sort of thing.
These need not be immediate effects , but build slowly over weeks or months of time.
how about a demonic equilivent of the gremlin?(see the wizards grimore)
Can you give a page number for that?
Which Rules edition , other than 5th?
If you like a little mayhem...
A trap that releases a demon. The demon was trapped in the device as:
A means of torment....he is now very angry
or
He was set to protect the area. Before he could be released, the area was destroyed...He is now out for revenge..thinking the characters were the cause of his 'Masters' demise. He might attack or just follow and cause all kinds of trouble...
-I would have it produce "Divine" Vis. This could have all kinds of effects...or maybe it gives off an Aura, and while it was in the Infernal aura, it was drowned out by the greater aura...but after its removed, the aura takes over...the Magi think all the botch dice and such are from Infernal taint.... 
If they didn't give it to the 'Brothers', I would recommend the Monks spirit would return and begin chastizing the Magi for their ungodly ways (never saying why he is bothering them....)
Other things...
Body that were dissected...some of them while still alive...
Human skins stretched out: some with Runes and such...some just being streched and dried.
Various offensive body parts...
Several types of torture devices
Summoning circles
Lists of names....some crossed off.
Various lab goodies...
A magical (device) that is very good at killing demons- A foolish 'demon' slayer thought his weapon gave him the power to face the group...
Just to keep the children gore up....
-
Braziers made out of the skulls of said children. Make them moan. Beauty for an infernalist's ears
-
Venomous and rabid rats
-
The usual pentagram in the ground with all kinds of infernal symbols carved on it. Make them make personality (brave, enduring, faithful....) or stamina rolls to avoid puking at the sight of it. or anything in the area, BTW. Can incapacitate them for 1-5 rounds.
-
Black blood can drip out of a stalagmite and be forming an equivalent stalagmite in the ground that appears to have trapped and tortured spirits when examined closely. Personality checks are in order here to see if the character is insensible enough not to puke and try to destroy it. it can be made into a corrupted source of vis if you want.
-
I like the blackmail idea. The characters have a dilemma: use it themselves to gain a profit (sinful, and make potential enemies) or destroy it. Will they turn down such a massive income resource? I would make most blackmail mundane, so that the magi can consider using it.
Hope that is disgusting enough for your saga 
Cheers,
Xavi
Blasphemous graffitti on the walls. If studied like a text, it is like a summa on Infernal Lore with a low quality and level. The graffitti have quotes like:
For a good time, summon Balphegor.
Chai-nog gives good ablation.
Dhoragg likes it in the maw.
A corpse that look disturbingly like one of the Magi.
A mannikin that is slowly being covered by bits of hair/skin/etc, that, when finished, will be a complete 'person' costume (like Buffalo Bill from Silence of the Lambs). Perhaps this is part of an infernal mystery initiation.
Priest's robes.
A journal written in dozens of different hands and from many different points of view and signed by different people, but with sequential dates, apparantly all written in this room.
I forget whitch page in the wizard grimore,but it was in the section on improving your labaratory.
does that help any?
OH no,
My wicked little post will NOT be hijacked by Abe. I humbly ask that no one respond to his temptations.
I just want to thank everyone for their fantastic suggestions. The adventure went swimmingly, and successfully creeped out everyone involved. I'll try and write up a more complete summary when I've got some time.
Eric