Where planets would be located in a “normal” solar system, the dyson sphere at the nebula’s center is surrounded by a vast field of asteroids and space debris. None of them have any significant natural resources. The largest ones have a measure of gravity and thin atmospheres, but all are barren. Some surely serve as hidden bases for those wishing not to be found.
A tiny but noticeable portion of the debris field are starship graveyards. Many of these are of unknown design. This has led to speculation that the nebula was over time discovered by various spacefaring civilizations wanting to plunder its riches, only for the would-be conquerors to meet their doom when arriving en masse.
The sphere itself (unimaginatively named Centerpoint by the early explorers who discovered its relation with its companion stars) is about the size of Jupiter, built around a white dwarf star massive just as much as our Sun, but no larger than Earth. It encloses the star almost completely, with only a single equatorial and four meridian-like rings perpendicular to the equatorial one cutting entirely through it and letting a bit of the starlight out.
Those who wish to approach the sphere must first get past its automated defenses. Any incoming vessel will be set upon by an array of blaster turrets. This response is proportional to the perceived threat. Smaller ships only get fired on by a couple turrets and skilled pilots can get them into low orbit, where the barrage ceases, without much trouble. A capital ship would be beset by hundereds, including much more powerful ones. A fleet would be met by the full might of the sphere, obliterating it completely.
The outer surface of the sphere mostly looks like any other space station; exhaust ports, docking bays etc, but no habitat domes. Those are found on the inner side, although there is also an Earth-type breathable atmosphere there. Starships may pass through the ring-like cracks in the sphere, and there are docking ports on the inner side as well.
The inside of the sphere is a tangle of tunnels connecting habitats, factories, hangars, warehouses, maintenance bays and many other kinds of facilities. The inner surface is similarly dotted, the endless metal occasionally interrupted by a habitat dome. Curiously, there are streams, ponds, rivers and even seas of fluids on the inside. Some of them are clearly the byproduct of the sphere’s functioning, but others seem tho have no rational reason to be there other than artificially being placed and maintained. Some are pure and drinkable water, some are saltwater, others are polluted water, and yet others are various other kinds of fluids, sometimes harmful and sometimes not.
The sphere is not inhabited by any sentient creatures, in the manner in which a starbase normally is. It seems to have been abandoned by its creators a long time ago. But there is still an ecology present. In and around the habitat zones, diverse plant and animal life survives. Aside from before unseen exotica, explorers have also noticed familiar life forms, poached from their planets of origin. While many parts of the sphere seem abandoned and disused, some indeed in a state of disrepair and blocked by debris, others are meticulously maintained by a diverse array of ever replenishing robots.
While some ignore any visitors not interfering with their duties, many are hostile on sight. During engagements, they appear to communicate with their targets in various languages, and while most of them are unknown, some are not. In such encounters, they have identified themselves as the Caretakers, and have been urging their targets to “return to their assigned cell blocks”. No known encounter has resulted in anything other than either the total destruction of one of the engaged sides, or the visitors escaping.
There is also other life present on the sphere besides the plants and animals. Some sections are infested by monsters. Others are inhabited by small communities of aliens seemingly transplanted from their native environments. While most of them are hostile, a few can be peacefully interacted with. If they lower their guard enough, they reveal that they do not remember how they have gotten there, only that they have either arrived recently, or have been there for generations. In most cases, they eagerly accept offers of rescue.
Who or what created the sphere, as well as when, why and for what purpose remains a mystery. The Caretakers do not reveal, none of the other inhabitants know, and no records remain. The various station controls use as of yet undeciphered pictograms and are usable by humanoids. Together with the rest of the station construction, they indicate that the creators were humanoid as well. What happened to them is a question many of the more scientifically-minded explorers actively pursue.
Lair Of The Chaotic GM 
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