They don't actually worship the demon in the Flame, at least consciously. Rather they hear the whispers of what they claim to the Flame itself and heed its purpose, the many different subjects mostly working alone but sometimes meeting when the purpose of their God calls for it, such as when an object has to be recovered by[subject A] and handed over to [subject B] because [subject B] has contacts that'll allow him to bring it to [subject C] without anyone noticing... continue as needed.
That also seems much more interesting than actual demonworshippers, who are clearly off their rocker. If you knew demons existed and lived in an eternal battleground [or trapped by the most powerful force of Good in existance] would you really spend your whole life serving it, being persecuted for your beliefs and then go to... oh, wait you still end up in Dollurrh.
Also, people have speculated that the Keeper of the Demon may in fact be the Keeper of the Flame, while Jaela is the Keeper of the Demon. Think of it, everyone thinks there can't be a Keeper of the Demon while the other one lives, and so no one would suspect it. And she's just a young girl, the demon doesn't want some super devout and powerful cleric, it wants someone weak and manipulatable.
On the outside, it looks like a perfectly ordinary Silver Flame fortress-monastery.
Now see, here's the bit that you've missed. It doesn't look like a perfectly ordinary monastery, IT IS a perfectly ordinary monastery. The demon isn't an individual anymore, it's become part of the Silver Flame. The Flame is made of good, neutral and evil.
And then we come to the point that you're looking for. Don't give them an evil temple to destroy, give them a sacred site to the Flame that's being used by a mad monk listening to the dark side of the flame. Give them a village priest that's gotten his parrish whipped into a frenzy and burning people (any race will do) at the stake to purify them "To fuel the Flame of righteousness". Make 'em cringe every time you mention there's a church of the Silver Flame in a given town. Show them the madness that can grip the zealot's mind.
That aside, what Goblin Pride posted up pretty much fits the bill. If you've ever seen the movie Fallen that should give you an idea of how to approach the Whispering Flame cult. Imagine a cell structure of individuals and small groups, each working on their own jobs independant of one another (and probably completely unaware), and yet all the various small actions they do contribute towards a larger picture. Can you imagine the scale of schemes, plots and horrors you could manage with access to so many people (basically any worshipper of the Silver Flame on Eberron with some dark in their heart), all working in unison without any of them ever realising...
The poor old Church of the Silver Flame, it's always Zealot central whenever I read anything about it :P .
As much as I would love to say it's not "zealot central", unfortunately it is. Any organization that can call on that many paladins is going to have a bit of a reputation for drawing fanatical nutjobs. (Paladins tend to fit that bill as well.)
Church of the Silver Flame
The group you are refering to are called the Tarnished (Faiths of Eberron p. 69). They serve, either willingly or by being manipulated, the Voice in the Darkness, the Shadow in the Flame or what's left of the teachings of the Anti-Keeper Melysse Miron who is locked up in Dreadhold.
The Tarnished are really like the Siths, seduced by the shadows of the Flame/the Dark side of the Force. However, unlike the Siths, it is less clear who is a Tarnished and who is a Purified since they all call themselves Purified, and might not actually know they have been seduced by the Demon within the Flame. The Tarnished is what true Purified call those tainted by the Shadow of the Flame.
So, to answer the OP's question. Beside the small paragraph on the Tarnished in Faiths of Eberron, I don't think there's any other material out there. You can always use the boards to shape your ideas with other members.
So, if I understand your post correctly, you have a temple of the Silver Flame that is overrunned by Tarnished. Is it an organized group, do they all know each other are reverring the Demon within the Flame or are they unaware they are being manipulated and tricked? Or is it only a normal temple with a cell of fallen priest that either willingly or unwillingly serve the Anti-Keeper?
As Goblin_Pride said, I'd go with a normal looking temple. However, it might have subtle, very subtle differences with a true, untainted temple. Maybe a DC 25 Religion check could make you notice such subtle changes.
Example of changes. The lighting might be a bit more dim, making the temple less bright than others. At first you could think it's because the sun never hits the windows right, but what if it was on purpose?
Another minor change could be in the artwork. Paintings of the Silver Flame could put a tiny more emphasis on the shadows of the Flame. The lighting in the paintings are less bright, as in the real lighting of the church.
Maybe there's a statue of the Silver Flame where you see the Couatl and the rajah battling in fire with the sword of Tira Miron plunged into them. But if you look more closely, you can see that the sword has only damaged the couatl, but not the demon. Or if you look at it with a precise angle, like from where the priest is saying his prayers, it is clear that when you look at the statue from that point of vue that the demon is the one winning the battle against the couatl.
Maybe that particular temple where your PCs are going was the temple of Melysse Miron, and she might have added to her church secrets that priests after her would discover and be converted like she was to the Demon Within.
You could also add the influence of a cult of the dragon below in the area of the said temple. I think that the Tarnished could be compared to one of those cults, and the proximity of such a cult near the temple might add to the weakness of the priests to battle their inner demons.
Just my two coppers.
Rekko
I noticed that the OP hasn't answered to anyone yet, but it shouldn't stop us from adding fluff to this interesting group.
Another element that could be thrown in is an agent or two of the Lords of Dust that have infiltrated the monastery in order to help sway it towards the "dark side." They could be followers of the rajah bound at Flamekeep working to get it released, or in it just for the chance of spreading chaos.
Goblin Pride, Rekko, I'll definitely use your input! An ordinary temple that's just a very tiny bit off will do nicely. I'll go and read up on Five Nations and RoE now...