First a foremost if you have played in my group I would request you stop reading now as spoilers abound. The one where the Rogue Trader managed to TPK the party by consistently rolling 99...

Now that's out of the way

I love stars of iniquity and regularly use it's system generator.

In my greatest system so far I've placed a world close to the sun with its own unique gravity. The explorers have so far crashed into it twice but worked out its artificial.

The planet's hollow, filled with machinery, and something is sealed inside.....

But I can't decide what.

My original thought was a C'tan bound by black holes, but knowing my players they will let it go to see what happens.

So any other ideas welcome.

A pretty big void kraken.

A necron tomb city with pretty pissed off guardians.

A matrioshka brain that questions their existance.

Load upon loads of gold... that stops being gold once exposed to sunlight because of space magic.

Crabpeople.

A whole new civilization of humans, who await the return of the emperor and the group has to prove to them that they are worth of speaking in his name.

An Eldar Webway Portal (either CWE or DE)

A Tear in the Fabric of Reality into the Warp, but is sealed (by whom and exactly how unknown)

A Time/Space Distortion (to the future or past, or maybe both and controlled somehow)

A End of the Planet/Solar System/Sector/Galaxy Weapon of Mass Destruction

Gremlins trapped in a golden cage

Eldar God trapped in a "cage" (always wondered where they went)

Thousands and THOUSANDS of Warp Spectres Trapped in a device (Pandoras Box?)

Hundreds of Enslavers trapped in a Stasis Trap

A xeno race in cyro statis that hid after being defeated by the Emperor

A SM Primarch/Marine(s) in Stasis

A Needle and a Hay Stack (but don't ever combine them)

Eerie and Truly Nothing But Utter Black Nothingness.

Dang, giant space Kraken was my thing....

Anyway, maybe a storage device built by the Old Ones containing a whole C'tan.

A C'tan storage device imprisoning an Old One.

The sickest sound system ever.

A teleportarium capable of transporting cites.

A device that is actually keeping the sun alive.

An auto-manufactorum that can make anything. Anything.

An armoury of Khaine or Khorne.

A library of everything that has ever happened.

Skub.

Your brand new car!

An ancient gravitic weapon left behind by the Old Ones, capable of pulling even the largest starship out of the warp and crashing onto it's surface.

A lost primarch whose stasis casket crashed there. Who knows what the primarch has done.

A stable warp rift, cocooned inside some ancient xenos device to keep it sealed. The device has damage to it however and its integrity is failing...

My vote goes to the necron tomb, chiefly because I've read a novel with a similar idea and it seemed very cool both as a threat and in terms of environment. Just make sure none of your players have read "Hammer & Anvil".

Other than that, combining both the auto-manufactorum and the human civilisation might work nicely, too. Perhaps the "natives" have grown complacent because the machine (an AI? *gasp*) creates everything for them? Or there is a sinister truth behind the paradisaic veil where said machine has become an oppressive tyrant dominating every aspect of the inhabitants' life (like a Forge World, but less obvious and with more terror). Or the Rogue Trader attempts to take/exploit the machinery and triggers a three-way conflict between the world, "parasitic" human natives, and the RT's own forces.

Primarch in stasis from one of the Lost Legions might also be an intriguing idea. It might be a little too epic for some tastes, or even a bit too obvious for a 40k RPG plot, but there certainly is potential to tap. Also, you could always make the identification as Primarch an error based on the characters' limited knowledge, and it actually is "just" one of the Primarch-looking "Marines +1" created during the Cursed Founding ... and perhaps waking him up was one of the players' worst mistakes ever, even if they might not realise it at first.

Regarding "AI" or "alien intelligences"...

Don't hide anything in the planet. It is more or less hollowed out, yes, but the actual "secret" is hidden plain in sight.

The planets landmass and parts of the water are covered with a fungus. Nothing extraordinary, there's plenty of planets where the flora is fungal based, right?

Well this fungus functions as a neural network for a meta-intelligence. Basically, the planets ecosystem is alive and doesn't like that humans are always coming around, digging around in it, then leave.

The planet features some settlements, all empty, like people one day simply left and vanished in the middle of things.

When they explore the tunnels, let crewmembers start wandering out into the wilderness, where they vanish without a trace. If they investigate and start cutting into the fungal growths, they find their crewmembers and the missing settlers. Examination of their bodies shows, that they've been infected by the fungus it somehow forced the abberant behavior onto the settlers and crewmembers.

For further fun, let this be an abberant strain of Tyranids, and when the characters are eager to leave, their astropaths begins muttering about a shadow in the warp and thats whan a vanguard fleet shows up.

For enhancement purposes: Play Sid Meiers Alpha Centauri.

Well this fungus functions as a neural network for a meta-intelligence.

Creepy. I approve!

http://www.extremescience.com/biggest-living-thing.htm

Reminds me a bit of Mass Effect or a certain X-Files episode, but there's nothing intrinsically wrong with stealing ideas from elsewhere - in fact, the entire setting of Wh40k was created on that very basis. The art is in making sure it fits into the greater picture, and that the homage is either not or too obvious.

Oh, and Avatar! A lot like that, but without giant blue warriors, and mechanized walkers, and an unnecessarily aggressive military, and inexplicable technology, and-

Wait a minute...

Seriously though, not a bad idea. That would be massive psychic presence, Navigators and Astropaths might get some serious headaches when the planet wakes up.

I always figured that Genestealers cults act as psychic beacons for the hivefleets, which is why the three big hivefleets are always targeting Terra somehow.

To the Hivemind, the Astronomicon represents the biggest buffet they'll ever have.

So, a adaptation for a living planet would make sense, and fungus is easily digestable.

I always figured that Genestealers cults act as psychic beacons for the hivefleets, which is why the three big hivefleets are always targeting Terra somehow.

To the Hivemind, the Astronomicon represents the biggest buffet they'll ever have.

So, a adaptation for a living planet would make sense, and fungus is easily digestable.

I'm unsure on that front. If they did project a psychic beacon then imperial psykers, especially astartes and inquisitorial ones, would pick up on it and go flush them out more often. The tyranid synapse is almost anti-psy as it blankets an area and limits astropathic communication. It would make more sense if a brood of genestealers acted with a low-level psy-dampening effect - psykers can still pick up on it but it's not as obvious.

Simple solution: The psychic field is in general not strong enough to be picked up or is camouflaged very well. Or there simply aren't enough sensitive psykers around to pick up every genestealer infestation.

We do know that genestealers can crop up everywhere, so it kinda makes sense that there either aren't enough psykers or their psychic effect isn't as noticeable. The Shadow In The Warp appears once a Hivefleet closes in and prepares to Om Nom Nom.

A planet. Because...

We heard you like planets... so we put a planet in your planar.

It's probably a matter of interpretation, and might even be different depending on the books you look at - in GW's fluff, it is indeed described as "a psychic beacon", and since it must be detectable by hive fleets many light years away it should also be in range of other Imperial planets. However, it could indeed be that for human psykers it is somehow indistinguishable from other Warp-based phenomena such as the rifts between realspace and Immaterium created by Imperial starships.

Or, to throw in another possibility, maybe the Imperium does (at least occasionally) notice these things, but by the time such reports are forwarded to the appropriate department and a defence effort has been organised, the hive fleet has already arrived in-system. It surely takes time to amass the necessary amount of troops to repel a full-blown Tyranid invasion ... just sending a few picket ships and one or two regiments from the closest world, as is done to counter raids from various Xenos or CSM warbands, might be considered a waste of resources. Either you react in full force, or not at all.

Have you read Dan Abnett's Eisenhorn trilogy? The third book in the series involves a hollow planet hiding-

----SPOILER WARNING-----

-the 'chariot' of the dead fourth Chaos God (the one before Slaanesh)...

Squats - they just have to be somewhere

Zoats - as above

Planetary system with a strange gravity planet closest to the sun, etc - matrioshka variant

Gateway to some dwarf galaxy near Milky Way

Gateway to Trazyns the Infinite shack of cool stuff

Bionic heart made of gold; it's traits unknown till inplantation

civilization of very small people, Guliver style

single, dead Jokaero with good-job-done smile upon its face

A planet. Because...

We heard you like planets... so we put a planet in your planar.

I'm actually starting to really like this idea.

Have the outer planet be some sort of dead world that looks like it has nothing of value, not even any minerals worth mining and even if it did it is far too close to the sun to be any good to anyone. However inside is a smaller planet with a thriving civilisation using the heat from their planet shield to power their civilisation.

Could be a lot of fun designing an ecosystem that exists entirely in darkness.