The Warp, bane and blessing the galaxy as it is, has been responsible for countless examples of strange and twisted phenomena over the ages. It is a little-known fact that it is capable of stretching into other universes and bringing in various flotsam into ours. For the most part, such things are strange and uncommon, and are usually localized, at best generating rumors that spread no further than the system they occurred in.
One time, however, it was different. On the world of Ghravov IV, torn between the forces of the Space Marines and Imperial Guard, Orks and Tau, one such Warp interdimensional anomaly occurred. It spewed forth a horror that bolter and chainsword failed to purge. The horror would spread fear, horror, and loathing even among a hardened chapter of Space Marines. It would make Orks shocked and the Tau make an exception for their policies. Through what was no doubt the blessing of the Chaos Gods, it would wreak untold havoc for such an individual.
What follows next has been drawn from transcripts and reports from the sides fighting on that world, although naturally it must be taken into account the mental affects this untold abomination had on those interviewed. I truly pray to the God Emperor in his eternal wisdom that such an event never repeat itself.