High-speed image of an Enforcer Object firing a beam of massive particles, as seen from a passing interstellar craft
For many thousands of years unexplained shipping losses in interstellar space have occurred on a regular basis, most of them caused by chance collisions with unusually large pieces of interstellar dust. Rumours of attacks by `ghost ships' and `foo' were common explanations for these losses, together with some blurred, low resolution images of mysterious objects glimpsed in deep space.
But thanks to advanced shielding technology, in 3455 the Matreiya, a Sophic League vessel survived such an attack, near Gamma Hydrae (where a number of other craft had previously disappeared). The Matreiya reported this event as a deliberate attack by an unknown assailant, an unidentified craft or object located in deep space, almost stationary with respect to the nearest stars, which fired a stream of massive particles towards them. The Matreiya was receding from this object at a very high velocity, but managed to get some clear images of it. The resulting images showed a ragged, half decayed-looking spheroid with numerous flabby protrusions, which was clearly artificial.
Three decades later on a follow-up mission the Explorer class ship Vajrayana found the object again, located more-or-less in the same place between the stars. As before the object fired its particle beam at the Vajrayana, a stream of crystal pellets which posed a real danger to any vessel moving at interstellar cruising speeds. In an attempt to disable the object the investigating vessel fired a UV laser at it, but the mysterious structure suddenly disassembled itself, hurling almost half its bulk directly at the Vajrayana. Thanks to advanced propulsion technology the Sophic League ship was able to avoid collision easily. The entire encounter lasted less than a second.
Over the next few millennia many similar devices were found, always between the stars, at least one light year from the nearest star and deliberately positioned on a direct line between two main sequence stars. These devices seemed to be deliberately targeting interstellar spacecraft. Most of the devices destroyed themselves, or were destroyed, without responding to any messages targeted at them. A few inactive examples were eventually discovered (more difficult to find as they were drifting well away from the spacelanes). The technology of these examples was found to be reasonably similar to the technology associated with a number of xenoarchaological remains, from the unnamed species HIE072CZE which seems to have disappeared 5.4 million years ago. Many of the remains of this civilization show signs of deliberate destruction, which is generally consistent with attacks by the weapons carried by the mysterious interstellar devices.
Eventually the abandoned homeworld of the HIE072CZE was discovered, orbiting TYC 3340-2314-1, 2910 ly from Sol. Collating all the available evidence the great xenoarchaelogist Macc Exxapno produced a new hypothesis, which postulates that the disappearance of this species occurred as the result of a conflict between the unnamed species and robotic devices of their own creation. In this hypothesis the mysterious interstellar objects were in fact some kind of defence system which became more dangerous than the enemy robots themselves. He called the objects `Enforcers', a name that has come into general use wherever these dangerous devices are found.
Unnamed species HIE072CZE themselves were somewhat similar to Terragen slimemolds or fungi, and their artificial enemies seem to have been very similar in form to themselves. The enemies sought by the Enforcer droneships were shapeless and protean, and it seems that at some point the droneships lost the ability to discriminate between friend and foe.
The technological level of this civilization seems to have been quite high, as the Enforcers are powered by a form of conversion technology and are very durable and efficient at self-repair. Even after five million years many examples are fully functioning. The Enforcer objects position themselves directly between two main sequence stars, presumably looking for spacecraft which had been subverted by the enemy devices. Because of proper motion, all of the stars in the former HIE072CZE empire have moved tens or even hundreds of light years in the last five million years, so the Enforcers have migrated throughout space to find new alignments. An unknown number of Enforcer objects are currently migrating through deep space, moving from one alignment to another.
As well as intercepting fast-moving craft by projecting pellets into their path, the surviving Enforcers will track slower-moving craft and sometimes match speed with them, possibly attempting to determine if the target is a carrier of the long vanished plague. For some as yet-unknown reason the Enforcer almost always decides that the answer to this question is affirmative. Most modern interstellar ships are easily able to defend themselves against the Enforcer particle beam weapons; but in many cases craft may avoid the straight-line route between stars, and the ancient devices that may sometimes lurk on those routes.