Hi there!

I , after a long time , finally managed to get my group together and host a DH game. But one of the players , the one with the most knowledge about WH40K (and also who knows me best) foresaw , at least partly my plan for the adventure.
Maybe I should give you a synopsis:
The Acolytes Inquisitor sends the Acolytes ,who for in- and out-game reasons have been separate for the pas two years (in-game) ,on Dusk in order to test their survival skills .Shortly before that he received a report from one of his allies about a strange energy reading outlining a temple which had never been registred before.The Acolytes ,dropped a couple hundred miles from the Temple have three months to get there , investigate it ,and come back to the drop-point.
Now , this temple is an Eldar Temple , but in it a Chaos Space Marine lies dormant ,a cult dwells there led by three sisters whose goal is to revive him , although the cult actually believes he is a Angel of the Emperor.
The group didn't get to the Temple yet but met a tribe who hinted that an Angel of the Emperor lies in it. My problem is the following : the player afore mentionned is convinced (rightly) that he is a Traitor.The whole idea of this event was to suprise my players and to pitch them against an enemy never seen before but now he expects to see a CSM. I don't what to do , whether to keep the adventure unchanged ,or replace him by a Loyalist who will go mad or not or whatever...
I'm aware this is not a very clear description of my problem so please excuse it , and if you have any ideas please submit them!
Also ,should anyone be interested in more details about this adventure , or the campaign it belongs to , I'd be happy to answer you.
Thanks,
B.Kaedran
P.S: the paragraph in bold letter indicates information unknown to the players.

What the Player is convinced of means nothing. The Player's PC does not share the same knowledge of the Imperium and its setting. Out-of-character speculation by the Player is all well and good, but to translate that to in-character speculation of the PC would require Tests based upon appropriate Lore Skills. And, unless there is any evidence to show a direct link to what the Player thinks and what the PC might deduce, Testing a Skill is nothing more than fishing for validation, and I'd say you have a clear case of a PC knowing far too much forbidden knowledge for his own good.

Maybe you could change the player's mind over time by having multiple encounters with tribesmen or cultists, all of whom believe that the marine is in fact a Loyalist. Use some of his knowledge against him -- for example, maybe you could introduce Blood Angels imagery or references to convince the player of the marine's Loyalist status. Unbeknownst to everyone, the marine is not a Blood Angel but instead is member of the Knights of Blood, a Blood Angels successor chapter who slowly went crazy and is now considered traitorous by the Imperium. See this link for more info and ideas: http://warhammer40k.wikia.com/wiki/Knights_of_Blood

Thanks for the reply Orpheo,
The PC is not aware of this , I just wanted this element of the adventure to be a suprise for excited players who where happily expecting to see an awesome Space Marine. This adventure is just a starting point for a much larger-scaled campaign and the presence of the Space Marine be it a Traitor or a Loyalist is not relevent for it . By that ,I mean that I can change the situation at will.
However, you have a good point ,one which touches a very hard part of role-playing : pretending not to know.
B.Kaedran

Covered in Weasels ,
I love your Idea , which my limited knowledge of the 41st Millennium would never have allowed me to come up with.
What I could see happening is that the Acolytes discover the Knight of Blood who at first would be a "normal" Space Marine . However as he spends time with the Cell he senses their corruption (for example my Assassin has 30-something CP) and he gradually goes berserk , turning against the Acolytes who will then be forced to bring him down , which will be a arduous task as I believe a crazed Space Marine must be quite the challenge.

Thanks ,
B.Kaedran

The other option is to 'reassure' them. Make out that it's a blood angel. Let them see the blood angels heraldry on the crypt/chamber in which the 'angel' sleeps.

Find him lying, in a stasis crypt, in a white robe, unarmoured with suitably loreal-esque blonde hair trailing. Make it absolutely clear he's a blood angel, giving his name, company, etc.

Let them start the process of reviving him.

Then the storage compartment next to them opens. It contains a suit of power armour painted black and tied with red saltires.

How about making him a loyalist marine, but one who has been asleep/damaged/adrift in the Warp for so long that he no longer knows or recognizes anything in the Imperium?

Make him have "heretical notions". E.g. surely the Imperium of Man cannot be run by mere mortals?

Basically, make him a good guy, with the wrong view on things.

The PCs shouldn't feel the need to dispose of an Imperial Adeptus Astartes, but they shouldn't be able to ignore his actions and opinions, which are obviously "heretical" compared to the wonderful modern day GrimDark of the 41st M.

Both the last ideas sound cool as well but wouldn't really fit in with the overall adventure ; that's my bad though as I haven't set it for you:
The Acolytes have been separated for the last two years (in-game) ,and are reunited on Dusk . Except the Assassin , (played by me ,even though most people don't agree with a GM Character) , who serves as a sort of spy in the group for the Inquisitor.
The Cell was sent there to face Dusk's dangers as a survival test but also to investigate this strange ruin recently revealed by energy scans .
This ruin is an Eldar Temple ,predating their Fall . It harnesses the power of the soul , and in conjunction with others would have been able to create an energy grid not only giving free endless energy but also allowing for Warp-like travel in the material realm . It could also have been used as a very powerful weapon , firing blasts never missing or raising armies of corpses . This is a broad description.
A Space Marine fell from a ship , after a fight (or whatever) and came down crashing . His power armor protected him but greatly injured and weakened him , thusly he was forced to take refuge in the closest shelter : the Temple.A cult then formed around him ,trying to revive him by prayers . Originally , I had thought that the three original priestesses tried to contact the Emperor by channeling their mind through the Chaos Space Marine , only to be corrupted and maddened by the very sight of Slaanesh . Their Daughters would have then taken over the lead of the cult and started ordering sacrifices in order to revive the CSM and to please Slaanesh . However the cult itself never saw the chamber where what they believe to be an Angel of the Emperor lies, and are true believers ,even if they have set a foot on the path to damnation.
Now , if we have a Space Marine instead ,there is a bit of a change.

What I thought could happen instead is we keep everything except the Chaos involvement. The Space Marine is revived ( the Temple feeds energy to it and every sacrifice helps) after a while ,a Knight of Blood. At the beginning everything is fine but then he starts sensing the foul stench of Chaos and prompts the Acolytes to help him cleanse the area . Once every man ,woman and child is slain , he still senses the Warp and tries to chop up the Cell.
I would like to give to the players a sense that there might be something wrong with the , for us obviously psychotic ,SM, that maybe these people are just unaware folk.

You're right. GM characters are universally abhorred. And for good reason .

Regarding your scenario, there are some things requiring a bit of polish IMO.

For starters, your Eldar temple is ridiculous. It seems like you only forget to add it spams Avatars of Khaine every minute.

There is no reason for such a powerful 'temple' nor would the Eldar have likely abandoned it. At the very least, they'd have stripped it since its so **** powerful. If you need it to sustain your space marine, fine. Give it some weird crystals and a blood stained daize.... Oh, and add eldar sentry guns and/or wraithguard as temple guards. Can't go wrong with sentry guns and unliving guardians....

Secondly, a space marine falling to the planet? Seriously?

Even if that were remotely feasible, his shipmates would have tried to find him, if only to harvest his geneseed. Or his ship would have been destroyed in the space battle and then the wreckage would have obliterated the temple since they were low enough for him to fall and survive....

To make this likely, I'd add a squad of space marines who died trying to find their missing battle brother to the mix. This is why the locals know there is a 'surviving' space marine in the scary temple (either they were interrogated by the space marines or they found the bodies when they moved into the area at a later date). Let a few locals carry jury-rigged astartes weapons and gear (maybe with the icons & colour obscured after so many years) so the players are further encouraged to expect a space marine.

The slaanesh cult corrupting the locals could work nicely. The three witches seems like a nice setup. You could go the obvious route and make them an orgy spamming cult or focus on some other aspect of Slaanesh. Maybe they admire beauty and perfection and kill all ugly things (including fellow villagers)?

As to reviving the space marine, the actual revival process needs both a sacrifice and something the cult could not provide (maybe forbidden lore eldar or tech use or something like that) to power the procedure. One acolyte must sacrifice an arm to make it work....

And then your end game would do nicely. The space marine awakens, feels the corruption all around him and starts a mini-genocide with the (enforced) assistance of the acolytes. Who are responsible because they raised him, even sacrificing an arm to do it. And then the space marine turns on them......