The Compilers are an offshoot race of the same ancestral race that gave way to the Einjel, along with the Viiska. They resemble the "Grey" alien stereotype of human ufology. Although they were created within the timeline of the Ancients much like the Einjel and Viiska, their status as an "Ancient" species is debatable as they remained in stasis tubes until well into the modern age (their evolutionary path is not entirely natural and their "constructors" never had reason to release them from their moorings). However this also caused the Compilers to completely lack any even remotely analogous form of culture and they seem to all other races to be nothing more than mindless, seemingly unstoppable killing machines.
The Compiler do have a limited relationship with the Viiska, since the Iskian Empire in which the Viiska serve occasionally deals with Crux, though both groups are wary of each other.