As it is a sci-fi setting, a small, highly advanced nanotech hive-device would work as an initially underwhelming, yet very powerful artifact. It would hold a small swarm of utility-fog style devices and act as both a control and manufacturing device for the swarm. It would have a limit on the mass of the nanobot swarm, probably somewhere between the size of a matchbox and a deck of cards; this due to limitations in its ability to control larger swarms. The swarm would be able to turn into essentially anything of the mass or smaller than the above mentioned limits.
The true power of the device, elevating it above the 'swiss army knife' utility obvious from the above description, is that it contains knowledge of how to create very advanced pieces of technology native to whatever race created the artifact. Tell it to create a generator and you may get a 1cm diameter, intricate device utilizing physics not yet known by man to create enough energy to power a moderately sized starship. While it can be used to create all sorts of handy devices, in the hands of those well versed in science, great discoveries may be made about the technology of those who created the device, as it has the capability of creating small examples of alien technology, ripe for reverse engineering. [extent of alien technology able to be created or reverse engineered would be primary balancing mechanic of the device, as GMs see fit]
The device monitors the amount of nanobots in its swarm, rebuilding more as needed until the maximum number it can control is reached. Any bots in the swarm farther than [insert arbitrary distance here] would automatically deactivate and essentially turn into harmless dust as the device lost control over them and replaced them with newly manufactured bots. These effects would mostly be for gameplay reasons to prevent the player from accidentally leaving their swarm somewhere, making the device useless, as well as limiting the power of the device to prevent the player from simply spawning 100 billion advanced alien starships.
The device could appear like all sorts of things, from a typical small stone with hidden panels/buttons to an elaborate box adorned with jewels and strange markings; whatever worked best for the setting. Upon being activated, but not instructed to create an object, it would emit what appeared to be a strange smoke, the swarm. Upon being instructed to create an object it was capable of creating, the smoke would coalesce into the object being created, with part or all of the smoke, depending on how much of the swarm was in use, disappearing. If it was told to create an object it was either incapable of creating due to size, it creates a small, useless toy model, as if to say 'you idiot, I can't make that!' If it was told to create an object about which it has no knowledge it tries its best, sometimes creating something like the request, but usually failing and creating some random item entirely unrelated to the object the user intended to be created.
Well... that's one idea anyway.