So in the course of my Dark Heresy game, I love to thread various McGuffins into scenes; an Imperial Guard chaplain who may have shown some Pure Faith talents (nobody could see what happened, but the Daemon didn't kill the unarmed old dude before someone else stepped in to finish it), a Jokaero that lives in their base (filching bits and things and returning them all shiny and chrome), a Hellblade the Dark Obsession player locked in a vault instead of destroying, recurring individuals such as Leonard Jechinnarus the Hereticus Inquisitor (a great guy who advocated their euthanisation after exposure to a rather massive Warp incursion, three party members are still plotting ways to kill him because of that), Rogue Trader allies, nobles in the area, etc. I find it gives lots of flavor and later plot-hooks (the various trophies the party takes do this too; aforementioned Hellblade, Neural Agonizer, Gauss Flayer, Ork Power Axe, Kroot Rifle, a Splinter Rifle, all weapons that have nearly killed the party and ended up in their possession/trophy vaults).

They recently fought an incompetent cult. I built up a lot of suspense when they saw the rash of kidnappings and the two bodies they found had multiple Chaos symbols etched into them, of different Chaos Gods. They were jumping to 'oh no, it's Chaos Undivided, they're working together to kill us all!' In reality, the cult just barely had a clue about what they were doing. Half the party showed up at the final session due to scheduling problems, and the three less-combative PCs wiped the floor (I say less because while they aren't as good as the main ones, the tech-priest has an Omnissian Axe, the Psyker Primaris has a Force Hammer and the Astropath has a Power Fist. It was hilarious watching them flail and miss with these super-weapons and cause massive structural damage). What they find in the basement of the manor (hidden in a secret door in the fire-place with a sconce lever, utter cliche) is a dozen cultists who almost all get butchered (one immediately gave up when they heard someone say they needed to take only one of them alive), the prisoners and the cult paraphernalia. Such paraphernalia includes:
1. A full 'daemon skeleton' was the prize of the collection. Yes, Daemons don't leave skeletons, it was a fake with mold lines still on the bones. Still bad, because it was 'accurate' for a skeleton of something that has no bones.

2. Obscura-laced incense.

3. Ritual implements of dubious quality; the circle was actually crooked, and had it been used bad things could have happened to the people.

4. Some macabre items (human bits, athames that killed people, etc.). Some of them were of some cult-value.

5. Grimoires apparently written by kindergartners, judging by the quality. The runes and script were misshapen or spelled wrong (which translated to the symbols on the cultists too), Out of thirty books or so, only six had any accurate information in them.

6. A pile of other 'evil' things that were really just fakes and trash sold to the naive.

7. A dozen emaciated prisoners kept horribly. Their stories/identities match up and they're loaded up for debriefing/purity checks.

8. A single, better kept prisoner named Roger DeWitt, a local noble they'd grabbed a while back. His story checked out with an initial background check, so they kept him at hand.

9. In a different section of the manor, there was a phial/urn made of crystal and fire opal. Extremely pretty. It is filled with ashes, and the label at the base says 'Ashes of a Thousand Son.' Too much ashes to be from a single normal person, so obviously there could be a bunch in there. There was a notebook nearby, which explains the owner bought it and was quite aggravated as it was obviously a fake.

The party crates up everything that needs further investigation, the fine china and a carnodon skin rug, then proceeds fill the place with promethium, burn it down, then have a bull-dozer come through and level everything. Roger disappears during the drive home, leaving an apologetic note saying he's sorry for lying, he's not actually a DeWitt. He wishes them a good day, too. (I have three different plot-lines on what/who he could be, but that's another tale). The phial was kept by the pyromancer Psyker, as it is pretty and apparently not dangerous (she didn't Psyniscience it either). It currently sits on her mantle.

You all probably get the significance of Ashes of a Thousand Son. All but one player in my group (out of seven, two currently not playing) are actually unfamiliar with 40K as a whole; it's all new stuff to them and they learn the setting from in the game. The last one, my newest player, got bug-eyes when he heard what it said, but acted in character. None of them have Forbidden Lores related to Space Marines, Chaos Space Marines or the Long War/Horus Heresy. The database they have access to mostly covers Xenos stuff. They have no clue what it is, so they've put a couple lackeys onto trying to figure out what significance the name has.

Now, I haven't decided a few things so far; for one, should it be real? If no, bad things can still happen. If yes, there's more options. The second bit is they pinged Thousand Son in an Inquisitorial database. That might turn up somewhere and cause a ruckus. Or a traitor lets it get to hands who'd like to get their mitts on it. Who knows? Whether or not it is real, attention might be headed their way. Advice would be greatly appreciated, folks.

A GM by my own heart. I love a good McGuffin. Especially one that not even I know the actual purpose of (there's a few sitting around in my game waiting for a flash of inspiration/cruelty).

As for the ashes...well...

You could make it a slow burn. Nothing happens at first, but slowly, spooky things start happening around the base. The psykers get bad dreams. The veil starts to weaken. Psyniscience doesn't get any pings off the ashes. What's going on?

If they players resolve to destroy/dispose of/hide the ashes, they find them missing. One of their servants has taken it and hidden it in the base. It's a mystery. Do the players suspect their servants? How will they work out which one has taken the ashes? Maybe they can't.

Just before it's time for the sorcerers/cultists/boogity-squad to show up (it doesn't have to be Thousand Sons - some other Chaos worshipper may have divined them) to seize the urn and torch the base, the characters find the servant in question babbling and scrawling glyphs that hurt the eye on the walls in their own faeces.

Well, one thing that has come to mind recently (and would be even more of a twist for my one experienced player) is to have the ashes be an unwilling Rubric Marine, a loyalist who got dusted somewhere else and is reconstituted out of the ashes as an apparently whole, healthy Space Marine. At that point, they've got a not-so-traitorous Traitor Legion Space Marine on their hands, with all of the risks that come from that angle (everyone who wants them dead, everyone who wants to study them, everyone who wants to shove him in a petri-dish, etc.). Imagine how Blood Ravens or Space Wolves would react to that, or the Deathwatch, or the Ordo Hereticus or Malleus? Or Chaos itself? Think of what would happen if Magnus heard about one of his poor Sons recovering from the Rubric?

On top of that, think of how the Space Marine would react. He's got a language barrier of 10,000 years, an utter change in the paradigm of the Imperium and a whole slew of things to deal with.

Well, one thing that has come to mind recently (and would be even more of a twist for my one experienced player) is to have the ashes be an unwilling Rubric Marine

Hah! If you're going to have a Space Marine in play, might as well make it an interesting Space Marine. It certainly opens a lot of doors, plot-wise, most of which are probably very painful for the PC's. I like the idea of SgtGoon's "slow burn" as a build-up to this being the conclusion, because once something like this gets out, I can't see it going anywhere but fast-paced and frantic (Space Marines tend to do that at the best of times, let alone a 10,000 year-old, not-a-Traitor Space Marine that every Imperial, Chaos and Xenos organisation of any significance will want a piece of).

I also like the idea of such a priceless artefact being in the possession of some two-bit cult in the back-end of nowhere; seems...fitting, somehow. Kind of like a certain puzzle-box that comes to mind or that jungle-themed game...

It is filled with ashes, and the label at the base says 'Ashes of a Thousand Son.' Too much ashes to be from a single normal person, so obviously there could be a bunch in there. There was a notebook nearby, which explains the owner bought it and was quite aggravated as it was obviously a fake.

The 'obviously a fake' is interesting - because that implies the owner had a reason to think that. Or, that he was influenced to think that.

A nasty trick would be a psychic 'taint' to make everyone ignore the vial. (which would, of course, retroactively explain why the psyker never picked up any 'spidy-sense' warning - she didn't explicitely say she was going to, and, of course, she took it home and essentially forgot all about it. As she was supposed to.

I agree that making it a 'slow burn' works well for the item itself, but I also agree that searching for "ashes of a thousand son" in an inquisitorial database is going to trip someone's radar. Most likely a Xanthite inquisitor, who'd know exactly what it is.

Which would lead to an interesting situation where the party finds themselves being investigated discretely but forcefully. Have one of the lackeys disappear, for example. He's being interrogated. If they find him, he's been telepathically scanned to the point his brains are leaking out of his ears.

Equally (later), have a gifted thief (death cultist level) break into and rifle the base. She won't find the vial - or rather she'll walk right past it without realising it because it doesn't want her to 'find' it - but she will in the process find the vaults. Which gives you the possibility of (a) leaving them in a situation where unknown parties know they have powerful and borderline heretical items in stock, and/or (b) her finding the Hellblade - and it not letting her put it down - and launching a psychotic attack one one or more of the sleeping acolytes. They did take Light Sleeper, didn't they?

And yes, if you want to draw in the actual thousand sons, remember that there are many factions - the 'core legion', the Brotherhood of Dust (Ahriman and his exiles) and plenty of renegades who left just before or just after the rubric.

At the same time, if you want to really twist the levels of paranoia with the player who knows, start adding in references to 'cyclops' - have cults being advised by a one-eyed mutant of unknown origin, etc.

Don't forget the Corvidae might come looking as suggested earlier.

Loyalist Thousand Sons with an odd habit of stealing every relic or interesting bit not bolted down could cause all sorts of wonderful problems.

A part of the reason the owner thinks it is fake is because he's an incompetent who 'tested' it by taking a pinch and snorting it to see what would happen (it returned itself to the phial). Nothing obvious occurred, so he thinks it was fake. Keep in mind, I played up the incompetence of these guys a LOT. That my players don't immediately double-check his work is hilarious.

The metaphysical slow burn is definitely on the table; the game is actually going to have some rather large... Distractions coming up, and a part of my idea has coalesced into the Ashes 'eating,' for lack of a better term, the ambient Warp energies to recuperate, causing more psychic phenomena but reducing the more dangerous aspects of what is to come. If someone (like a psyker) were to wear it, I'd basically let it eat all the bad psychic powers sent their way like an incubating egg soaking up warmth (while putting out Psychic Phenomena (no Perils, just Phenomena) every time it happens). This would also speed up the Space Marine's return.

The main reason the players haven't tipped their hand yet is because they're using the Inquisitor's 'private' database; he has two, one he allows all Inquisitorial agents to access and one that only his people are allowed to use. They stick to the latter for security, but given it doesn't have any information on this topic they might have to start extending feelers. If they'd talk to the local Hereticus Inquisitor he'd have the information, but they really don't like him.

I do like the idea of people in their retinue going missing, though I will have to really come up with something for a thief to infiltrate their base. We're talking former Imperial Guard command center that's been refurbished; there's a grand total of four doors (main, hangar, two garage) thicker than a person. Impersonation of someone returning from their weekly trip to town would be the best option... But then they have to operate in said center without getting spotted. Small window of opportunity before someone notices something is awry. My party is somewhat paranoid, and the command center was acceptably positioned AND abandoned when they acquired it. It took a lot of kissing up to the Mechanicus (a Heretek and an Arch-Heretek slain with evidence to prove it AND a lost Titan recovered) to get it to the current level.

I definitely would not let the thief wield the Hellblade, as that is currently crooning to one of the PCs in their sleep (she picked up the Dark Obsession malignancy, I ruled it was for Daemons, she's why the thing is in a sealed, warded room instead of the party trying to destroy it). I don't want to give anyone another reason to want to get rid of the thing, and I don't think it'd want to ruin the opportunity to get a phenomenal wielder turned from the God-Emperor. Not to mention having to go from the basement vaults to the top floor quarters past... A lot of people. No way they get there with the PCs still asleep.

Cylopean-led cult will be in there too, perhaps with Chaos Space Marine support at some point. It will be quite easy to work in my current campaign direction... Magnus will really want to know how one of his Sons recuperated when he finds out it happens (I'm actually thinking that if it does happen, he'll immediately feel something occurred and will hustle his ass over quite quickly).

So, my next question is... What will the reconstituted Marine come back with? I mean, he could have all his war-gear, or fall out of the sky naked in a Terminator pose complete with the fancy orb and lightning effects. He's going to be Psy Rating 5 Raptora (my head-canon says the 'potent' psykers who survived the Rubric were all 6+), so plenty of telekine abilities to make up for a lack of weapons/armor. If he doesn't have his gear, I might even have him try a ritual to summon an old possession of his (Force Staff maybe) and it comes through all covered in tentacles and Corruption, which he'd immediately destroy in disgust. Or he summons his armor and gets it in a dozen pieces, half of them broken and the other half apparently pulled out of a working suit of Power Armor (sucks to be whoever was wearing it just now!). Without his gear though, he's just a super-human, naked dude with mind-powers and no identifying marks. If he has his armor, the group will have heraldry and other things to work with, but are probably more likely to immediately start shooting before the apparently Warp-infused Space Marine gets a chance to shoot them. Maybe having the event occur while they're less on guard will help...

So, my next question is... What will the reconstituted Marine come back with? I mean, he could have all his war-gear, or fall out of the sky naked in a Terminator pose complete with the fancy orb and lightning effects. He's going to be Psy Rating 5 Raptora (my head-canon says the 'potent' psykers who survived the Rubric were all 6+), so plenty of telekine abilities to make up for a lack of weapons/armor. If he doesn't have his gear, I might even have him try a ritual to summon an old possession of his (Force Staff maybe) and it comes through all covered in tentacles and Corruption, which he'd immediately destroy in disgust. Or he summons his armor and gets it in a dozen pieces, half of them broken and the other half apparently pulled out of a working suit of Power Armor (sucks to be whoever was wearing it just now!). Without his gear though, he's just a super-human, naked dude with mind-powers and no identifying marks. If he has his armor, the group will have heraldry and other things to work with, but are probably more likely to immediately start shooting before the apparently Warp-infused Space Marine gets a chance to shoot them. Maybe having the event occur while they're less on guard will help...

Or he can be literally 'dust in a tin'. Most Rubricae are automatons, but there are a few cases where they aren't, and there have been a few cases where a thousand sons sorcerer has been killed several times (Madox is the Bill King Space Wolves, for example).

I wouldn't necessarily have him 're-flesh'. If he's been flambéed by the Rubric, getting his skin (and tentacles) back would take some doing.

By comparison, I agree on making him a Telekinetic. Because if it's a guard command centre, I'm pretty sure there will be some ceramite, plasteel and adamantium plate lying around (i.e. forming the doors and walls) and having it be wrenched out of the walls telekinetically and essentially cold-forged into the (rough) shape of MkIII astartes powered plate should be a disconcerting thing to watch. You could always have someone notice a few wall panels be loose in the room where the vial is before the 'main event'.

Plus, a telekinetic has (a) no real problem if not given access to weapons and (b) able to protect himself briefly with a kine-shield whilst busy with "some assembly required".

Well, my point is that he won't be a Rubric Marine anymore, and is refleshed (without visible mutation that would cause immediate extermination, at least). Because refleshed is huge. This isn't something the Acolytes or the Marine is doing, it's something that happens as a result of local shenanigans, possibly a once-in-a-billion-years event. Imagine how the Inquisition would feel about this, or the Deathwatch, or the Imperium, especially if he professes his loyalty to the Emperor and the Imperium. Imagine how Magnus would react. They're all going to want a piece of this Marine. They're all going to want to deny each other access to his pieces. Who knows what kind of plot Tzeentch has put together with this act? I really don't want the decisions my players make to be easy, and a Warp-animated tin-can or tentacle beast do weigh the scales so as to give them a simple choice.

In that case, you're definitely going to want to have Ahriman lurking somewhere - narratively, if not necessarily figuratively; whatever's allowed this to happen is basically a means to achieve his life's work; to undo his mistake with the Rubric.

Having the marine be apparently loyal is an interesting one - it should, as you say, provoke interesting choices. Although simply being innocent or loyal is no particular guarantee of safety where the Inquisition is concerned....