The Urd-hel are a mechanical race. The original creators have been lost to antiquity. However, the technology behind the Urd-hel is nothing short of incredible.
Physically, an Urd-hel appears as a small black box that is about the size of a rubix cube. However, when an Urd-hel activates, it can project forcefields that allow it to move about and interact with the world around it. This is augmented by holotechnology that allows it to take on any appearance it wants, and signal replicators that will fool every known scanner as well. In truth, very few individuals know when they are dealing with an Urd-hel.
When an Urd-hel dies, whatever "magic" that powers the black box is lost. Scientists that have studied inert Urd-hel find a solid cube composed of refined Viridite. It is unknown whether a live Urd-hel is composed of the same material or not since no Urd-hel has ever allowed itself to be studied.
Additional Ideas (1)
Urd-hel in the Galaxy
The Urd-hel are not well known in the galaxy. Most of the time they masquerade as other species. The one weakness in the Urd-hel's ability to mimic others is the fact that, being mechanical in nature, they have no psychic presence. Psychics are rare in the galaxy so widespread testing for Urd-hel is not feasible.
The Urd-hel seem to have a plan for the galaxy, but no one is entirely sure what it is. Most Urd-hel that are discovered have insinuated themselves in a position of influence. No one is sure whether that is because Urd-hel are drawn to those positions or because those positions have higher security and thus are more likely to be caught. What is known is that wherever an Urd-hel has been discovered, conflict has been a major feature of the recent past.
Urd-hel are capable of imitating and replacing people, and this is a fairly common method of infiltration. The complex heuristics contained in their cores allow them to copy the mannerisms and mindsets of the subject, and advanced predictive algorithms allow them to appear to have memories and experience that they do not.