The Inhibitors from the Revelation Space Universe were a machine race left over from a massive war that occurred between the first civilizations in the Milky Way galaxy. They had evolved from an organic race to cybernetic to fully machine.
They were non-sapient machinery, "post-intelligent", and capable of self-replication. They had black cubes of "pure force" that were immune to conventional human weaponry. Their task was to inhibit the spread of intelligent life beyond solar systems and suppress star-faring cultures by planting triggers and waiting for sapient life to activate those triggers. One such trigger was activated by humans, leading to the Human-Inhibitor War which was fought at a great cost to human life.
By the early 27th century they were able to push back the Inhibitors and establish an Inhibitor-free zone around human space.