My name is T.S. Luikart and I've been involved with Dark Heresy pretty much from its inception… this would be the point where most game designer's notes start waxing on about how they became involved in said project, offering up various endearing personal anecdotes showing how much they love said RPG, etc. However, you guys have been waiting for this game so long I suspect that even if I announced I was a devout worshiper of the Architect of Fate and that playing Dark Heresy would invariably lead you to drooling damnation, you would likely still read on. Thus with the understanding the Changer of the Ways now has a trifling lien on your soul, let us talk of psykers.
Psykers for those few of you new to the Warhammer 40,000 universe, are those souls who can channel the power of the warp — that unseen dimension of emotion–charged energy that lurks beyond the skin of realspace, into a wide variety of supernatural effects. By force of will, psykers can start fires, move objects, read minds and divine the future. Their unnatural abilities bear a heavy cost though — by drawing upon their psychic powers they become a potential conduit into the physical universe for both uncontrolled energy from the immaterium (one of the warp's many names) and a number of dangerous predatory entities that lurk within the warp.
Psykers have few friends, with good reason. Even their closest allies have to accept the fact that they may one day be forced to put a bolt round through the back of the psyker's skull and if (Ha! When!) that day arrives, a second's hesitation born of undue sentimentality could mean doom for an entire world. With all this in mind, I set about laying the groundwork for how psychic powers would operate in Dark Heresy: they would be dangerous to face but also dangerous to use.
When psykers invoke their powers, they roll a number of dice varied by just how powerful of a psyker they are and aided by the force of their wills. Yes, it is possible to be powerful in psychic ability, but weak willed. A very dangerous prospect, to be sure. If they beat the Threshold of the power they're invoking, it activates. Many psychic powers grant a range of additional effects if the psyker's roll beats the Threshold by a significant amount, allowing a powerful psyker to get far more out of a given ability than a lesser one can. It also ensures that psychic abilities remain useful throughout a Player Character's (PC) entire career and since psykers get access to only a handful of abilities (and what hard choices that can make for!) this was an essential component of the system. I tried my best to make every Psychic Discipline tree desirable and my fellow designers helped to do a whopping good job of making them all amazingly useful. I'm especially proud of the Divination Discipline. Traditionally, I've noticed Diviners tend to get the short end of the stick in RPGs. Wait until you see what Divine Shot and Precognitive Dodging will do for you.
Now the dangerous stuff; when any of the dice rolled to invoke a psychic power come up as an 9, a Psychic Phenomena occurs — roll d100 to find out what terror has been unleashed. Everything 74 and below is bad, but not fatal. 75 and above necessitates a roll on the Perils of the Warp table and the potential for very bad things to happen…
The Perils of the Warp chart would be the one wherein the psyker can seriously injure and/or kill himself along with everyone around him (for instance, all the other acolytes in the vicinity) in a furious burst of Warp energy. The various entries on the Perils table are the work of many designers; I'm fairly fond of Blood Rain though Mass Possession is good for a laugh. For the GM, I mean.
In the end, it wasn't enough that Non Player Characters should fear the psyker - every other PC (and the player behind him or her) had to as well. I suspect that some groups might have serious discussions about whether or nor they're willing to tolerate the presence of a psyker… which is just as it should be.