The Pheraean Rift is a spatial anomaly in the Eridani Sector. It appears to be a particularly dense dust cloud that occludes the light from stars on the far side of it. More interesting, however, is that it is almost completely impossible to cross in the warp. Navigators who have tried note that they lose sight of much of the surrounding warp currents, and then the Astronomican starts to dim. There are no recorded instances of a successful crossing, although there have been seven documented attempts, three of which resulted in the loss of the ship, and the rest of which turned back. The cause of this warp effect is not remotely understood, due to the impossibility of exploring the region. It is unknown if it is a naturally occurring phenomenon, or the result of xenos artifice. No xenos presence has ever been known to emerge or go into the region, though, and inevitably there are infinitely more theories than facts on the subject.
Historically records have shown that the features of the Pheraean Rift have been a boon to pirates and raiders, giving them a place to withdraw to in order to avoid pursuit due to the local forces obvious reluctance to venture too far from the light of the Astronomicon. This has led to a constant background rate of incidents in the system surrounding the Rift. If all of these systems were able to combine their knowledge they would now be able to see that this raiding has been in steady decline for more than a century as fewer and fewer raiding parties return after retreating to the Rift, however each and every system has put down any short term reduction in events of disappearance of an otherwise long-established raiding group as simple generation to generation variation or them moving on to other systems. The reports of ghost ships, and abandoned settlements are also passed off as the results of internecine squabbles.
Most recently, a number of small outposts in the region have started to fall silent. The few that have been investigated have been revealed to be completely abandoned and where any remains of habitation of industry have been found the degree of entropic decay suggests that have not been in use for centuries or even millenia. That communication may have occured with these outposts as little as months or years ago is put down to the vagaries of warp communication in the vicinity of the Rift.