WARNING to players of my AoW campaign (Wilson and Dave I'm looking at you) don't read this! Spoilers below.
In my campaign we are almost finished TFoE. The PCs fought their way through the maze and made their way into the inner temple of Vecna. They killed two of the acolytes, and reduced the allip to low hp before it hid in the wall. Being afraid of the allip (after losing a lot of Wisdom) they now stand huddled, back-to-back in the centre of the "inner sanctum" (room 26).
While they were in the maze, to make things more interesting, I had a hissing, menacing, disembodied voice (the Faceless One) mock them as they progressed. At the moment we ended the last session, with the group all together in a circle in the center of the sanctum, it seemed the perfect opportunity for a little creative license to heighten the tension, so I had the disembodied voice say "Thank you for cooperating, and being exactly where I want you!"
So, my conundrum? How to make good on the Faceless One's disembodied threat and play upon their position in the sanctum.
I decided that, since it is a temple of Vecna, the god of secrets, and the former study of Vecna himself, it stood to reason he might have some means of extracting secrets. Thus I came up with the Tome of Secret Extraction, and the new spell "Detect Secrets".
At the start of the next session the PCs will suddenly be bathed in green eerie light. Looking up they see arcane symbols painted on to teh ceiling. These symbols create a magic circle 20' in radius from a point directly below. The symbols are connected to the Tome of Secret Extraction, which is activated by reading the incantation on the first page (a standard action). The tome can be used from up to 60' away (the Faceless one can use it from his laboratory next door). Targets within range must make a Will save vs a DC of 10 + 5 + the INT bonus of the person using the tome (in this case +5 for the Faceless One). Anyone making the save feels like something tried to probe their thoughts. Any target who fails the save has a secret plucked from their mind. This secret appears written in the Tome in green ink with the name of the person. It is written in the same secret coded language used by the Faceless One to correspond with Theldrick, which can be deciphered using the information found in the Faceless One's laboratory.
The means of making such a tome, and the circle of symbols that go with it, are themselves a secret known only to Vecna, and so potentially lost to the world, so this particular tome can only be used within the temple in TFoE.
However, worshippers of Vecna might discover the spell that is a part of the item, which can be used without the tome, but only against one target per casting. The spell appears below. Please let me know what you think. I have based it on Detect Thoughts.
Detect Secrets
Divination [Mind-affecting]
Level: Clr 5; Sor/Wiz 5
Components: V,S,M/DF
Casting Time: 1 Standard Action
Range: Close (25' + 5' per 2 levels)
Target: One subject with Intelligence 3 or greater.
Duration: Instantaneous
Saving Throw: Will negates
Spell Resistance: No
Casting this spell against a target with intelligence greater than 3 plucks one secret from the mind of the target and makes it known to the caster. The caster "hears" the secret in his/her mind either in a language spoken by the target, or in images if the target has no spoken language. If the caster does not speak the same language, he/she must record the secret phonetically and have it translated. The secret is revealed in one sentence, and hearing it does not necessarily impart understanding of it to the caster, who must interpret what is heard. For example, the caster might hear from a target the secret "I killed Felbar", but won't necessarily know who Filbar was, how he or she was killed, or any other information. The caster can not control what secret he/she gets, or question the target.
Material component: A blank piece of fine paper worth at least 1 gp, or a blank page torn carefully out of the caster's spell book. The paper is consumed in the casting, disappearing in a small burst of flame that does not cause combustion of other materials.
I've only just started my new AoW campaign, and the group hasn't even left town to truly starts Whispering Cairn yet, but consider this idea pilfered!
I like the idea, particularly with the restriction that the spell can only be used in this temple.
I'd caution you to reconsider letting the PCs actually learn the spell itself. There are a number of NPCs later in the path that have secrets which shouldn't be revealed too early. And some of those NPCs have numerous servants/minions who may know their secrets, and will not have high Will saves. Therefore, I'd recommend that you either eliminate the potential to re-discover the spell, or if you must include it, at least make it a ritual with a long casting time (which should let you manipulate things more easily to avoid its use at inconvenient points later in the path).