Lord Dunsany noted that it is unwise to summon that which you cannot dismiss, but considering the amount of email I've been getting lately, you all don't much care.
So by all means, let's talk about psykers and their powers in Dark Heresy. So first a full confession for those tuning in recently - I designed the overall structure by which psykers work in 40K (loosely based on Wizards from WFRP) and wrote a fair number of the powers, but by no means all. What follows herein are my opinions on a number of psychic Talents, powers and a variety of optional rules (and rulings). These things are my opinions and in no way "official", feel free to use or ignore anything I have to say on the subject. As to why some things were changed, my general answer is that a fair number of my original write ups were a bit more complex (covering many of the loopholes you all now love to debate over) and that a number of qualifying statements throughout the powers were modified and/or dropped in what I perceive as an effort to make Dark Heresy a bit more user friendly.
Rather than try to follow all the varied threads on psykers, their abilities and the varied arguments being bandied about, I figured I'd start this one, which I promise to check in on regularly.
So first, general rules:
You can only manifest one psychic ability a turn... The sole exception that I allow is Resist Possession which I allow to be used as a "Dodge" of sorts, 'cause lets face it, a possessed psyker can end your campaign right quick... the thought of which still kind of entertains me but the appearance of mercy staves off all sorts of complaints.
You are required to roll at least 1 die to manifest an ability - I wrote another set of posts on my ideas for alternate Invocations to get around this rule. It is, however, the original "official" position if you were still wondering.
So Psyker Talents. Let's talk about my personal daemon first: Corpus Conversion. The original idea behind CC was pretty straightforward: Like Tetsuo screaming out his rage, like any number of 40K protagonists that spurt lightning from their eyes and bleed from every pore, I figured the ability to take damage for raw power made perfect sense for psykers. CC as it is presently written allows far to much power for little risk. I have done the following in my personal games - note in advance that this is bloody harsh, but it is working out splendidly:
All psykers start with Corpus Conversion, it is now a psyker Trait. Johnny come lately psykers learn it soon after they pick up a Psy Rating 1 (or higher). CC allows a psyker to burn 1 permanent point of Toughness for an additional d10 on a Power Roll. They can do this up to their Willpower Bonus on a given turn, e.g. 45% Willpower, 4 WB, burn/sacrifice to the warp/whatever up to 4 Toughness points in order to get up to +4d10 on their Power Roll.
Now you know why psykers tend to be scrawny little bastards, yes?
Favoured by the Warp. Drops one's chances of ever having a "Perils" roll into the bog. This works for some games, not others. You may wish to change it to the following: You may remove a single 9 from any Power Roll. The removed die neither adds to the Power Roll, nor triggers a roll for Psychic Phenomenae.
A few Powers:
Constrict - The current favoured whipping boy. I agree that the "must" makes Constrict far to obnoxious for its low Threshold. Change the "must" spend a Full action to a "may" if the target wishes to "throw off" the Constriction. I think a Willpower Test to ignore the fact that you're being choked to unconsciousness / death is appropriate.
Fun Fact - Constrict was originally intended to allow a variety of uses, you could use it on ruptured blood vessels, e.g. you could "constrict" a blown off limb, automatically stopping Blood Loss.
Divine Shot - Like "Call Item" DS originally required a weapon that the psyker was attuned to - a personal sidearm or some other such weapon. It doesn't open a "hole through the warp" or any other such nonsense. It's an old Eldar trick that mankind has absconded with. It originally had a line about truly impossible shots, while still "hitting" may have their effective damage reduced to 0, e.g. the light from a Hellgun blast gets reflected off several surfaces and a mirror before briefly dousing its "target" in harmless light.
You have my permission to slapped players who declare that they're "attuned" to their "personal lascannon".*
Note that you also originally had to have a physical line of sight to the target, none of this "Far Sight" b.s. I do think that optical assistance, on the other hand, would work, e.g. Diviners carry cool looking brass telescopes for a reason...
...as do Pyros as Fire Storm also requires a "physical" line of sight to manifest on target.
Holocaust - The wounds caused by Holocaust can only be healed by time, psychic abilities do squat to restore them. That may seem harsh, but consider this: Daemons generally laugh at most mortals, including the bulk of psykers. Even if you are such a badass that you can take a daemon on with a reasonable expectation of killing it and surviving the fight, the corrupt bastard is laughing at you because all you're doing is destroying its manifested form and banishing it back to the warp where it will spend the next century or so contemplating the horrors it will visit upon your family line for the next umpteen generations. Holocaust burns spirit as well as flesh: anything killed by a Holocaust is warp-dead as well as physically dead. Psykers that can manifest Holocaust scare the **** out of daemons...
...not necessarily a good thing, but big bragging rights, eh?
Psychic Blade - Why the hell would anybody manifest psychic force visibly? One of the bad guys in my personal game was a serial killer that would lop the limbs off his targets in broad daylight (well, gas light, hive and all) and nobody would know who did it. Yes, you can ambush the crap out of people with PB. Yes, it is horribly nasty. It's also missing a crucial sentence: "When rolling a 95-00 while attacking with a Psychic Blade, a stray thought causes the psyker to attack and automatically strike his nearest companion. If there are several, the psyker strikes the individual he is emotionally closest too. If there is no one in range, he automatically strikes himself."
Projection - Yep, you can do all the other Telepathy powers while projected. You can Project, have your fellow Acolytes catch your suddenly catatonic form, "walk" right over and shove your ectoplasmic fingers into your chosen target's brain in order to Mind Scan them, and then head back to your body double quick.
So why is this not powerful beyond all get out? Well other than the sidebar "Closer Than Flesh" remember that there are a number of entities that will be interested in either making an incautious psyker's projected form their personal chew toy or taking their recumbent form for a stroll. The Imperium (along with many Xeno races) have a number of materials / wards that can block psykers from just waltzing in and possessing the local Planetary Governor for kicks.
Basically, it was perfectly left a little vague so a Telepath could work her magic, but a GM that feels abuse is in the offing has a number of tools to encourage said psyker to be careful.
*I accept no personal responsibility if you actually do slapped a player, even one that really, really deserves it. I might buy you alcohol at GenCon if you are of legal age though.