system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] what exactly is a Psyker and why are they so feared?
gpt
They can psychicly manipulate the warp to effect real space. They are common conduits for demons to enter real space. An untrained psyker is highly likely to fall to chaos and attempt to slaughter or subjugate those around them. The distinct strength advantage they have over regular humans means they are a constant threat. 

The abilities of a psyker typically originate from a mutated strain of DNA known as the psyker gene.