I already posted an adventure idea in WFRP and I received many good ideas of it, so I'll try it with this one as well.

This would be the second part of a Campaign. The characters are looking for one of their relatives (in my party an assassin looking for her sister disappeared long ago).
In the first part, she followed a prophecy, more specifically a verse. She managed to free a most unusual psyker girl (who was secretly enslaved in a machine by heretics to control the whole infrastructure of a city), who helped her to puzzle out another part of the verse, what directed them to a forbidden world.

The second part would be their trip to the world and free her sister.
There was a war long ago around the world, that involved xenos technology, gravity charges. Many of them (around 100) still active and in orbit around the planet, but the planet's upper atmosphere contains flammable gases, and the gravity charges attracts them, so they make their gas balls around them. Additionally the gases emits light, so it seems that the planet is having about a hundred suns. That will make the advance to the planet a bit hard.

The party is having a comissar ally from the previous adventure, who is a necron lord masked in human skin (from a previous adventure). They will hit the surface a bit hard and scattered along the surface, losing all their allies (the psyker, the comissar).

This is where I would need more exact ideas. I'm thinking about involving the Thousand Sons chapter.
The planet itself is a hot world, with lava flows, cracked red ground and such. As they arrive to the planet they'll meet a sorcerer from the Thousand Sons, but he'll lie that he is a librarian from a space marine chapter who lost all his men (they wouldn't recognize that this is a chaos space marine). They'll travel with him and he'll poison their mind with the teachings of the warp.

The trip will be hard I think I'll like to involve at least an attack of a thousand sons' homonuculus or something.

At the end of the trip the'll arrive to a castle build on vulcanic rock where the assassin's sister is held. I involved previously the Churgeon from Edge of Darkness as well. Maybe they could hold the sister there because of some grand sacrifice or an experiment or something. Maybe they wan't the psyker girl because of some reason.

I wan't the comissar to reapper at the very end of the advanture, when the necron uncloacks himself, but I already has an idea for that.

So what do you think? Any ideas?

If the world is very hot and lava and stuff, you should call for stamina checks to resist the heat while going about business. Otherwise, make them loose fatigue. Happy the pc who has resistance to heat. Checks could be about any 4 to 8 hours doing anything but resting, as hard or easy as you deem appropiate.

Damnit it!! Never manage to edit inside a huge quota

Why is "it" spying on them?

He is seeking revenge, for they revealed their tomb planet and brought the Adeptus Astertes on them. He wan't to crush their dreams, bodies and very souls.

Are they travelling on different ships, do they sleep in very different quarters or will they "drop capsule" in livepods?

They are on the same ship.

I am not sure if you really could be fooled this way, unless the sorcerer uses some kind of magick trick. Their would be major chaos taints on him, for sure. Ever thought about exchanging him for a Sorcerer of the Alpha Legion? As far as I know, they are much more into "tricking, posing and infiltration".

I want to stick to the Thousand Sons. Why would they discover him as a CSM? He is having a bright, power aromur, maybe some horns but thats all, nothin really suspicous at the first glance. They are thinking that he is a Librarian, so a psyker, so the mumbling about the powers of the warp wouldn't be suspicous as well I suppose. Of course he could use magic trick as well.

Honestly, this starts to sound like an "mash them all in". Necrons, Sons and the Logicians (oh my!) I would suggest you leave the Churgeon out of it. A group of a chaos space marine chapter ("Sons" or "Alpha") and a Necron in Disguise should be enough. Should it not?

Noo It could be a longer adventure, and I wan't a great conspiracy amongst these powers, but I don't know yet why or for what goal.

What is the rest of the planet like if it comes to "native life"? None? Remainder of a civilisation? Alien or Imperial? Anyone still out there? And why are the CSM there? A forward base? Has the planet "a very thin barrier to the warp"? Resources they want or artifacts they are searching for?

Well, I wasn't thinking about such things. Maybe the planet could hold native lifeforms, maybe lost civilisation. That's why I need ideas.
The CSMs are working on something, in alliance with the Churgeon and the Necron Lord and I suppose they need a specific person for that (because of psychic abilities maybe?)

OK, here again to be visible:

Why is "it" spying on them?

He is seeking revenge, for they revealed their tomb planet and brought the Adeptus Astertes on them. He wan't to crush their dreams, bodies and very souls.

Are they travelling on different ships, do they sleep in very different quarters or will they "drop capsule" in livepods?

They are on the same ship.

I am not sure if you really could be fooled this way, unless the sorcerer uses some kind of magick trick. Their would be major chaos taints on him, for sure. Ever thought about exchanging him for a Sorcerer of the Alpha Legion? As far as I know, they are much more into "tricking, posing and infiltration".

I want to stick to the Thousand Sons. Why would they discover him as a CSM? He is having a bright, power aromur, maybe some horns but thats all, nothin really suspicous at the first glance. They are thinking that he is a Librarian, so a psyker, so the mumbling about the powers of the warp wouldn't be suspicous as well I suppose. Of course he could use magic trick as well.

Honestly, this starts to sound like an "mash them all in". Necrons, Sons and the Logicians (oh my!) I would suggest you leave the Churgeon out of it. A group of a chaos space marine chapter ("Sons" or "Alpha") and a Necron in Disguise should be enough. Should it not?

Noo It could be a longer adventure, and I wan't a great conspiracy amongst these powers, but I don't know yet why or for what goal.

What is the rest of the planet like if it comes to "native life"? None? Remainder of a civilisation? Alien or Imperial? Anyone still out there? And why are the CSM there? A forward base? Has the planet "a very thin barrier to the warp"? Resources they want or artifacts they are searching for?

Well, I wasn't thinking about such things. Maybe the planet could hold native lifeforms, maybe lost civilisation. That's why I need ideas.
The CSMs are working on something, in alliance with the Churgeon and the Necron Lord and I suppose they need a specific person for that (because of psychic abilities maybe?)

Look like it not working...

I edited out the quote, looks like things are (mostly) fixed.

Nagash said:

Because of the taint of chaos. Take a look about the changes the "taints" can have on a pc when he amasses more and more corruption points until he "drops out of game due to corrupton". A full blown CSM (especially a wizard-psyker of the "Sons") will be over this line tens and hundreds of times.

But I agree that the "Sons" are far cooler Still, give him some kind of "illusion spell" that guises his taint. The psyker of your group (you have one, right?) will "feel" that there is a power going on... but this should be reasonable for the scriptor he is posing for.

Talking about planet & inspiration, I will answer AFTER fetching my copy of the now-arrived "Creatures Anathema"... which I will fetch RIGHT NOW.

There are many ways to run a campaign, and your ideas are very different from mine, but I hope you can find inspiration in my comments.

Every group of players has their own unique understanding of how the universe is, and what works in one might never work in other group.

I for one cannot really understand what your adventure idea, gains by involving the Thousands Sons or Alpha Legion sorcerers.

As I read your idea, you wanted to have some entity corrupting/tempting your characters while they investigate something in a remote/alien freakish strange place. And then you start jumping through hoops to make it a CSM sorcerer.
Why not create your very own non-CSM sorcerer to do exactly that? You could easily create him in a way that it would make sense of him to try and corrupt the players. Their very own cadre of inquisitorial acolytes would be the wet dream of many chaos followers (or warp entities) after all. It would even serve to make the plot more believable. You could give him an interesting background story, (like a fallen acolyte or even inquisitor), and a personality that will really make him one the players will love to hate. In short, make it personal. I think that would be a more interesting story than a chaos space marine sorcerer.

The novelty of anything - even space marines wears off, if you use them too often. In Dan Abnett's Eisenhorn, the characters see a CSM, and the encounter is very memorable exactly because he uses space marines very sparingly. So I would save CSM for something truely remarkable.

If you really wanted the involve CSM, I would use them as the next step in an escalation of power. Players could discover that the aforementioned sorcerer was an ally/ or was trained by/ was plotting against CSM (only to fail, as he would be unlikely to get the better of 10k years old CSMs sorcs anyhow). So the CSM would be the very thing one badass sorc is afraid of.

With the disciple of the dark gods, sorcerers don't need to be space marines to be really hardcore.

A necron out for revenge? Even revenge is a feeling, and as I understand it Necrons does not "suffer" from that. They are far too inhumane to have feelings of any kind being strange undead machines.
I cannot find any appalling reason In your adventure Idea why the players has to encounter a Necron. I would lose the necron in the adventure, or alternately use a necron tomb - place as backdrop for the adventure. That could create a situation so desperate that the tempting offers of a sorcerer might seem like the appealing option. If I was forced to use Necrons, I would make certain they came across as having nothing remotely human about them as to make them special, otherwise they might as well be replaced with something else that makes more sense for the story.
That was my comments on your ideas. I like to do things in a different way, but then again your players might think the adventure was great because it involved a CSM sorc and Necron. That is just difference of play style.
Good luck with it.

Thank you for your advices Any ideas concerning the different plots in the adventures and the conspiracy in it?