So, some of you might remember my thread about a Rogue Trader version of Flying Dutchman.

Which I did do and it went pretty well. Not as well as I had hoped, though.

Now I have the idea for a treasure hunt in a Dyson sphere.

First off, is this doable in any way?

Second, what species should be the creators of the sphere?

It would take a while one thinks.

Some unknowable alien civilization that returned to dust long before man built his first crude hovel. Or maybe the Imperium if you chose something a little more pauldron shaped...

I don't see why it wouldn't be doable, but you should remember that a Dyson Sphere is absolutely enormous in scope, far larger than any habitable planet. Also, it may be interesting to have the "ground" (that is, the outer shell as perceived from the inside) essentially be a massively thick, multi-layered labyrinth, transporting energy 'round-planet and so on and so forth.

Second, none of the major known ones, really, unless you want it to be archeotech. Humanity certainly had (and arguably has) the technology to construct a dyson sphere, and probably did at some point during the Dark Age of Technology. The necrons/necrontyr certainly have/had the technology, but then arises the question as to why they would make one, or keep one, or if they did, why it's not a massive necron hub devoid of life.

All the other major specieses is unlikely to have made one or wouldn't have any interest in making one.

So either ancient human, or unknown xenos, I'd say. Could be fun to populate it with primitive humans that have regressed, thinking that the dyson sphere is the entirety of the universe, and the fully automated systems God.

Honestly I can't see Humans doing it. Even if they theoretically had the technology and a substance sturdy enough to serve as a building material, the sheer amount of building material required and the time it would take to construct is ridiculous considering most human scale space ships are measured in "mere" megatonnes. The entire sum total of our solar system's mass, including all celestial bodies, asteroid belt, and the various rings, and even presumably the Oort cloud and scattered disk, is apparently sufficient to make a Dyson Sphere 8-20cm thick depending on density (not pretending to be smart here, just curious so I googled it). And that's just literally everything, not all the unobtanium (or whatever), just everything.

So you'd have to produce enough building material to rival the contents of a few dozen solar systems, and then move them into one place, before you can even construct the thing. Tens of thousands of years to put together probably.

That's very true.

Hmm, would a ringworld the size of a habitable planet work better?

Oh you could totally do a Dyson Sphere. Stuff on ludicrous scale is definitely within the idiom of WH40k, I just don't see it as being a human made thing.

Necrons would have the bloody minded persistence to make such a thing, maybe eldar? The Old ones obviously, and the Slaan might have the level of super-technology. The webway might work since you could use it to simply teleport the materials a piece at a time through the webway to the destination as each piece of material is finished.

Other ideas. Worm World. A world(Ice?) or planetoid (sorry pluto) with no molten core, honeycombed with tunnel after tunnel. Hope you don't get lost...

How would the PCs actually get inside the sphere?

Forgive me if I'm wrong here, but isn't there actual mention of a Necron-constructed Dyson sphere in some of the fiction? It's speculated to be a prison for the Outsider C'tan, as I recall. Then again, that was OldCron fluff, so its validity is a matter of personal opinion.

Apparently that's below the galactic plane.

You could also consider using something akin to the shield worlds from Halo, which give similar effects on a planet-sized scale. Alternatively, consider having the dyson sphere around a cooler star, such as a brown dwarf, where the habitable zone would be be at a much smaller radius (while still huge compared to searching a planet.

Uh oh, here comes Errant to throw cold water on everyone's day. Quick has the idea there, but IMO, for the wrong reason. That stellar remnant tells you how old the construct is; it's not a determinant of its size. What engineer is going to build a forever civilization around a star that's already almost dead?

You say you want a Dyson sphere or ringworld, so I'm assuming you've read Niven. That's where people tend to get this inspiration. Do you recall how Louie Wu is always reminding himself of the scale of The Ringworld? It has the surface area of 3 MILLION Earths. That's not a sandbox, and you will run into scale problems, the same ones Larry ran into while writing his book, the same ones that prompted Ringworld Engineers .

Now a Dysnon sphere has a surface area of 550 MILLION Earths, given a 96 million mile radius, and that's pretty well unimaginable.

Hey, knock yourself out. I've played RPGs a long time though, and I can tell you that the players make the game, not the setting. Save yourself some grief and make an interesting sandbox for your players. Don't try to get epic before the game even starts. You're creating a recipe for self-disappointment if the game doesn't pan out.

Either way, good luck.

Actually, because their smaller formation size reduces the rate of interior fusion, brown dwarf stars are the longest lasting and most stable stars available - exactly what an eternal civilization would want to build around (besides not needing as much material). The size of the ring or sphere would be smaller because the star puts out less energy, shrinking the habitable zone.

Of course, you're still talking about thousands or millions of earth's worth of space, so it might not really matter. Just don't expect you players to 'explore' it any more than you expect them to 'explore' the entirety of a planet. (1 million times less, actually ).

Planets are huge, and most adventures only go to a millionth of a percent of their area. This will be even huger, but fundamentally isn't all that different. You still using some kind of massive remote sensing equipment (like your ship's augers) to find things to tell you where to go. So it's perfectly doable, but it's really flavor text & background, not anything that will fundamentally change the construction of the adventure.

Just be cautious of exploitation by your group - colonizing or salvaging from this has "broken game PF factor" written all over this.

Yeah, the size is an issue. That's why I'm considering the ringworld.

Which would be more like a Halo in size, which has a surface area of 318 million square kilometres.

For reference, Earth has a surface area of 510 million square kilometres if you count the seas.

EDIT: Whoops, it's actually 3,180,000 square kilometres.

I investigated Dyson's spheres as a place to set a game some time ago but the scale of the thing blew my mind completely but I did think that a Dyson's swarm might be more managable as each satelite doesn't have to be as immense and some might even just be lumps of raw material that were put in place for future development.

You also have the added bonus of being able to give the individual satelites extremely complex orbits as each satelite might influence the orbit of several other satelites. That means you can have ancient booby traps resulting in collapsing orbits or collisions... heck it doesn't even have to be a trap as such, perhaps the whole thing has lost calibration over the millenia so that even the explorer's ships landing can cause issues.

The Dyson Swarm brings to mind the rosette of the Pierson's Puppeteers, the Fleet of Worlds (from whence I'm pretty sure came the idea of the Craftworlds).

What you want is not a Dyson Sphere but a Dyson Swarm or a Dyson Bu bble, with a bunch of independently orbiting, synchronized plates. Make it made by some unknown xenos, and make the whole solar system completely devoid of any other matter (including oort cloud) than the Dyson bubble.