Captain Tuurok Castar of the Marines Malevolent has just received a crash course on why you do NOT mess with ponykind... in both the present, and a dark future that must not be.

A Conversion Bureau/Warhammer crossover with a Cosmic Horror spin. Also a Grimdark brutalization fic... in which the ponies are the ones doing the brutalizing.

No explicit gore, violence, or non-consensual sexual activity, but this is all the worst elements of 40k and the Conversion Bureau combined, and non-con body modification is endemic to these sorts of fics, so tags are present just to be on the safe side.