We're playing in Warhammer 40k Dark Heresy 1st Edition world. I'm a psyker and I have many psy powers which are stated as half focus. Our GM has not had much experience with psykers. He says I can only "cast" once per turn, but that doesn't seem logical to me.
How many psy powers can my psyker use per round of combat? Can you point me to the relevant rules? I'm a novice in WH40k, and I'd like to understand what's going on and have rules to point my GM to if they were mistaken.
You can Focus Power once per Round
Per the Dark Heresy 1st Edition errata, currently located at https://images-cdn.fantasyflightgames.com/ffg_content/dark-heresy/pdf/darkheresy-errata-v3.0.pdf
The first paragraph in the Using Psychic Powers section starting on page 161 should include the addition: “Making a Focus Power Action is the psychic equivalent of a Standard Attack Action, and counts as such for purposes of determining what else a psyker can do in a round. Therefore, a psyker who uses a Half Action power cannot make another Half Action Standard Attack on the same round. A Psyker may manifest only one ability per Round with the exception of the Resist Possession power. A Psyker is required to roll at least one die to manifest an ability.”
So no matter what, you can only ever perform the Focus Power Action once per turn, as it counts as a Half Action Attack (even if the power isn't an attack), and there are already rules in the Actions section regarding the inability to perform multiple Attack Actions (don't have my book to properly cite this, but the errata clearly states only one Action per Round).
The change in the errata was made to prevent Psykers from using both Focus Power and Half Action Standard Attack Actions in the same turn. The Action section in the combat chapter of the core rulebook (specifically under Half Actions) should have some sort of wording that states you can't perform the same Half Action twice in a round. You can't Standard Attack twice for example. You technically are taking a separate Full Round Move Action, not performing two Half Action Moves.
The usage of Psychic Powers is detailed on the core rulebook (page 161):
To use a Psychic Power, a Psyker directs his will into the sea of the immaterium, pulling forth power from the warp to create an effect in realspace. To do this, the Psyker must make a Focus Power Action.
This action is explained a few pages later (page 166):
Focus Time: This entry describes the length of the Focus Power Action required to manifest the power.
On this same page, we have a table listing all psychic powers in the book and what Focus Time they require, which go from Reaction, to Half Action, to Full Action.
Lastly, on the Playing the Game chapter (page 193), we have the following:
Focus Power (Varies)
This Action is used to manifest your Psychic Powers in combat. How long the Focus Power Action takes is dependant on the power being manifested.
So the kind of action you must use depends on the psychic power you want to use.
Under normal circunstances, you are only allowed one Full Action per round, so if you use a Full Action power, you cannot use any other power on the same round.
However, the combat rules state that you cannot make the same Half Action twice in the same round (page 188), which limits you from using your Psychic Powers (by using the Focus Power Action) to once per round:
Note that you may not take the same Half Action twice.
Even if he was wrong, you shouldn't attempt to prove him wrong, that is disruptive to the game. Just point him to the rule outside of the game, and accept his judgement.
As already noted by ShadowKras and Kommissar, you can only use the 'Focus Power' Action once per regular Round. This means you can only do a normal spell-cast once per Round.
One thing to remember, however, is that while you may only be able to cast one Psychic Power per Round as per the rule stating you cannot use the same Half Action more than once per Round (in this case the 'Focus Power' Half Action), you can sustain some powers over more than one turn. A Power that can be sustained lasts for 10 Rounds before needing to be sustained, which is affected by simply passing a Free Action 'Focus Power' for that Power. Furthermore, the only penalty for trying to cast and sustain multiple powers in 1e is that you get -4 to Power Rolls to cast while sustaining 1 spell, -8 while sustaining 2 spells, and -16 to cast while maintaining 3 spells. You can't sustain more than 4 powers at once.
Here's the entry regarding sustained Powers from page 164 of the Dark Heresy 1e Core Rulebook:
Sustaining Psychic Powers
There are a number of Psychic Powers that are worth “keeping on” for an extended length of time. Powers that can be used in such manner are referred to as being “sustainable”, which is noted in their entries. Sustaining a Psychic Power for an extended period of time is very taxing and only the most proficient Psykers can accomplish it. Every 10 Rounds a power is active, the Psyker must make a Power Roll to maintain it—just as if he were rolling to manifest it. Unlike a normal Power Roll, this is performed as a Free Action at the start of the Psyker’s Turn and cannot invoke either Psychic Phenomena or the Perils of the Warp.
Sustaining Multiple Powers
A Psyker can sustain a single Psychic Power without much effort once properly activated. However, all Power Rolls the Psyker makes while sustaining a single Psychic Power are reduced by 4. A Psyker can sustain multiple powers, but all Power Rolls get progressively more difficult as he must expend more and more of his power on maintaining the powers already in effect. A Psyker sustaining two powers reduces all Power Rolls by 8 while a Psyker sustaining three powers reduces all Power Rolls by 16. It is not possible to sustain more than four powers at once. These penalties to the Power Roll also apply to sustaining powers already in effect.
This is why powerful psykers are scary; they can buff themselves and/or others, debuff one or more enemies, and/or throw out insane amounts of armor-bypassing damage per Round.
