The Humble Gunpowder Mage - Shotgun Psyker
 Psyker Build - Warhammer 40k: Darktide 

The humble "double barrelled shotgun wizard"

Are you tired of purgatus staff and/or force great sword ? Do you still want to delete hordes of ennemies with the press of a button ? And more importantly, do you want to be able to scream "I CAST DOUBLE BARRELED SHOTGUN!" at your strike team before unloading into a mixed armor wave or elite patrol ?

If so, this build is made for you. 

Note that this isn't in any way a meta build. No soublaze shriek to delete everything, no force greatsword's warp slice, or telekine shield. Just you, scrier's gaze (which is incredibly strong, don't get me wrong) and a good old shotgun.

TL;DR : leveraging True Aim along with the Crucis Mk XI Double-Barrelled Shotgun blessings Man-Stopper and Flechette will allow you to proc consistent infinite cleave* bleed blasts, especially with a fast finesse weapon (which is why I chose the tacaxe for my melee weapon of choice)

*Man-Stopper's will actually cleave through anything until it hits carapace or a bulwark shield.

Ranged weapon

This is a pretty easy pick there. Crucis Mk XI Double-Barrelled Shotgun, like all shotguns, has access to both Man-Stopper (infinite cleave on critical hits) and Flechette (6 bleed stacks on crit). Combine that with True Aim, which give you a guaranteed ranged critical strike after 5 weakspot hits, and you get a consistent "delete everything in this general direction" button, as long as you don't hit carapace, or a bulwark shield.

For the perks, I usually pick Flak and Maniac for breakpoints into elites, ragers and specials. You can switch Maniac for Unyielding if you want better ogryn time-to-kill, this is down to preference.

Note on cleave

Man-Stopper doesn't state this explicitely, but it actually sets your cleave attack value to infinite, this is not an exaggeration. Yes, this includes cleaving through monstrosities (more testing needed for captains and lieutenants' voidshields).

However, the game has an innate "abort attack" mechanic built in carapace armor, which will stop any attack from going through. Do keep that in mind when unloading if there are crushers, bulwarks or scab ragers mixed in your ennemy wave.

Ranged alternative

You can also opt for the Accatran Mk IX Combat Shotgun (with the exact same blessings setup) as a good ranged replacement. It has reduced spread compared to the double-barreled shotgun, so it wont cleave as wide a path when firing into a horde, but it trades it for much better elite sniping accuracy, ranged damage, and a special that will apply burn stacks. 

Which means that the Accatran combat shotgun, when combined with Perilous Combustion and Flechette, will actually allow you to apply 3 status effects at once! It is the better boss damage weapon (burn has a 1.5 damage multiplier against Unyielding), but you lose the cool factor of the double-barreled shotgun imo. Very fun choice too though!

Additional notes

1. In spite of the pretty bad damage fall off of the double-barrelled shotgun at range; mechanics like minimal pellet count, and of course, the bleed from Flechette, allow you to use it to snipe specials pretty consistently. Even better if you proc Kinetic Flayer on your shot.
2. The double-barrelled shotgun has really good stagger. It will actually stagger a reaper out of its attack, great for suppressing gunners too.
3. True Aim stacks up from any weakspot hits, not just melee ones. This means that you will often rack up 2 or 3 True Aim stacks when unloading a Man-Stopper crit shot into a group of ennemies

Melee weapon

Any fast finesse melee weapon will do. 

I chose the Atrox Mk VII Tactical Axe for its incredible blessing choices, namely Brutal Momentum (mandatory), and Shred. Your melee weapon's primary goal is to clear up chaff that doesn't justify shooting (poxwalkers, grunt waves, etc.), while building True Aim stacks really fast. The tac axe is a really good match for this exact reason, since its attack pattern allows for easy consistent weakspot hits, with a good push attack for horizontal cleave still.

For the perks, I pick Flak and Unarmored, since I'll mostly be hacking into chaff hordes, and I want to clear those as fast as possible to return to more priority targets. Unarmored can be swapped for Carapace, since it is the one armor type that will give you trouble with this build.
Though keep in mind that the tacaxe special attack has insane stagger on weakspot hits, letting you stop a mutant in its tracks, or a crusher mid-overhead. Takes a bit of practise, but is a good way to safely engage in melee.

Melee alternatives

• Combat blades (knives) : have access to Uncanny Strike for handling carapace, lose a bit of safety and horde clear. I'd recommend Riposte as your second blessing
• Duelling swords : also have access to Uncanny Strike and Riposte, and more reach than the combat blades, but less mobility

Ultimately, this is down to preference. You could even run slower weapons, but you would lose mobility and fast weakspot hits.
Do note that with the shotguns having subpar mobility, your melee weapon of choice has to be your movement tool.

Curios

Straightforward. 3x toughness curios with the following perks:

• 3x 5% Toughness. You want as much toughness as possible, which you will be able to regenerate insanely fast
• 3x 4% Combat Ability Regeneration. Scrier's gaze is insane, having it up more often is a straight up DPS boost.
• 1x 5% Health. I always run this on Psyker for Havoc reasons. This puts you above the 100 health threshold even in H40, and prevents you from instantly dying to poxbursters when at full health

This leaves you with 2 perk slots to use at your leisure. Gunner resist, Ally revive speed, Corruption resist are all good picks here.

On all of my builds, I usually run at least a +3 Stamina curios and Ally revive speed, but with Warp Ghost and Kinetic Deflection, Psyker is actually insane at blocking and clutch reviving, letting you bypass stamina, and go full toughness.

Talents

This is a pretty standard Scrier's Gaze build, with a few alterations.

The key node here is True Aim, for reasons I already discussed. Guaranteed ranged critical strike, need I say more?

Blitz

I go with Brain Rupture as my default choice, for 2 reasons:

1. Access to Kinetic Flayer, means free damage burst every 12 seconds, allows easy specialist sniping with the shotgun in a pinch. This is one of those talents that you start keeping a track of unconsciously after a while
2. For some reason, since crushers spawn was reduced, the director likes to spawn waves of bulwarks at me. This build doesn't have an answer to a shield wall advancing on the team, but Brain Rupture can cover that to some extent

Otherwise, Assail is a decent choice, mostly for peril-maxxing, which this build lacks. Using Assail would allow you to maximize the DPS potential of this build, by keeping up critical peril most of the time. I don't like it personnaly, but it's a very valid choice.

Aura

Go for Prescience. You need all the crit chance you can get, for reasons I'll go over later, and you already have insane cooldown regeneration at higher difficulties thanks to Psykinetic's Aura. Hot take : Seer's Presence is useless in most Psyker builds.

Combat ability

Scrier's Gaze. This is a gun psyker build, no other ability will give you the benefits of Scrier's Gaze. I always take Precognition with Scrier's Gaze for the additionnal finesse damage, as well as time considered spent inside Scrier's Gaze and linger for 10 seconds. Keep in mind that finesse is both weakspot hits AND critical hits, which means... well pretty much all of your attacks (tacaxe headshots, and shotgun True aim crit shots).

Keystone

Keeping in line with the same reasoning as before, Disrupt Destiny is your only valid choice. Raw damage, along weakspot and critical damage, and speed and toughness boosts. What more could you ask for?
My safe keystone modifier recommendation is Lingering Influence, to keep up your stacks of DD between engagements, but Perfectionism can also be a valid choice for Havoc 30+ where tension mechanics allow the director to spam waves after waves of ennemies non-stop.

Mandatory nodes

I won't delve too deep in here, this is all pretty standard Psyker building 101.

• Mettle and Warp Expenditure are your 2 main toughness regen nodes. In particular, Mettle doesn't actually require you to kill ennemies, and allows you to regen toughness by whacking a boss, for example.
• Lightning Speed, By Crack of Bone and Warp Splitting are your melee enhancing nodes. By Crack of Bone in particular, allows infinite Scrier's Gaze uptime when fighting hordes.
• Empathic Evasion. One of the most broken talent across all classes. As long as you have something to hit, you can ignore all ranged attacks. 
• Psykinetic's Aura. Similarly broken, especially in higher diffs with lots of special spam. Will get your ability back up before you even notice it's on cooldown.
• One with the Warp and Just a Dream are very good survivability nodes, synergizing well together (25% flat damage reduction, plus 33% toughness damage reduction above 80% peril)
• Warp Rider and Warp Ghost for more damage, and to keep peril as high as possible. Remember that this build doesn't have easy access to peril generation, so less passive quelling is very much needed to keep your peril buffs up as long as possible.
• True Aim, for reasons already explained, and Kinetic Deflection. This last one gets you two main things : 1, it lets you block for pretty much as long as you want -you won't ever get your guard broken- and 2, as a byproduct, you'll be able to clutch revive even in the thickest horde.

Liberty nodes

This gets you 3 talent points to spend where you want. Here are a few good options:

• Usually, the utility of Perilous Combustion is hard to pass up for me (especially in higher diffs where DoT and elite spam make this a huge damage boost, so I invest into Quietude to reach it. 
• Perfect Timing is +15% damage pretty much all the time, also very good. 
• Vulnerable Minds gives better breakpoints into ogryns and better boss damage. 
• Surety of Arms can compensate for the somewhat slow reload of the shotgun, if that's something that you find lacking.
• Anticipation  is very good for better dodges.
• Warp Speed (Scrier's Gaze modifier) feels amazing if you can keep SG up for a long time.

Overall, I'd recommend experimenting with what fits your playstyle best.

Playstyle

This build requires you to play aggressively. You're not a soulblaze psyker hiding behind your frontliners. You have to  be up front and personal with ennemy waves, for several reasons.
First, you need to find things to hit to proc Empathic Evasion. As long as you hit those sweet critical hits, you won't have to worry about ennemy fire. This talent rewards you greatly for pushing ennemy lines. Just keep in mind which situations are "push-able" and which aren't. Some rooms are just not possible for you to rush in, you'll get stunlocked before you can get a hit in.
Second, hitting things will stack up True Aim for you, which will then allow you to use your shotgun to maximum efficiency.
Third, you are actually pretty tanky! One with the Warp and Just a Dream allow for small mistakes when engaged in melee and will prevent you from taking too much chip damage when comboed by two ennemies. Additionaly, with all the toughness replenishment you have access to (as long as you hit your crits for Mettle procs), your toughness should be back up almost instantly as soon as you take a hit.

Using the shotgun

The shotgun is your "delete" button. It can be used to clear a mixed horde for you to advance or retreat or to decimate an elite patrol that's nicely lined up, but don't neglect its sniping potential. Use it to stagger or kill a special before it becomes a problem for you and your team.
Ideally, you want to wait for those 5 True Aim stacks before firing, but it isn't always possible, especially when you just need to leverage the stagger potential of the shotgun (a reaper is gunning you down, a trapper is about to shoot her net, etc.).

Be especially mindful of carapace armor. This is the shotgun's Achille's heel, as it will stop your shot from going through anything else entirely. If you can, try to position yourself at an angle where the carapace ennemy won't block your shot, unload, then switch back to melee to quickly dispose of the carapace threat.

Regarding positioning, when possible try to engage or lead fights in tight spaces, where the shotgun will be able to shine by clearing whole corridors.

Melee playstyle

Your melee weapon is your chaff clearing and mobility tool. You will be using it most of the time, switching to the shotgun only for a quick True Aim shot. 
Use blocks push attacks proficiently. Stamina isn't a problem for you with Kinetic Deflection and you actually want to be blocking attacks constantly to keep your peril up.

Disrupt Destiny

Be aware of ennemies marked by Disrupt Destiny. You want those 15 stacks up as much as possible without puting you in sticky situations. The damage boost is significative (at max stacks you get +15% flat damage, +30% crit damage and +37,5% weakspot damage. Hit a critical weakspot, and you get all 3 boosts in one).

Short tangent into finesse damage boosts

In case you don't know, all finesse, critical or weakspot damage boosts only affect the portion of the damage increased from a non-crit, body hit. What this means:

• For the Tac Axe Mk.VII, taking the heavy 2 strikedown: it has 136 base damage into Carapace body. Hit a critical weakspot and that becomes a 610 damage hit, so a 474 damage difference. With 15 stacks of Disrupt Destiny, you get the following boosts:

  • +15% flat damage: +610*0.15 = +91.5 damage
  • +30% crit damage: +474*0.3 = +142.2 damage
  • +37.5% weakspot damage: +474*0.375 = +177.75 damage

  For a total of +411.45 damage, which give you hit a whopping 1021 damage! For reference, an Auric Crusher has 6500 health.

• For the Double Barrelled Shotgun, hitting weakspots is a bit more inconsistent, so take a critical body shot on a flak target. This bring a basic hipfire shot from 992 damage to 1170 damage. Applying the same damage calculation as before, 15 stacks of Disrupt Destiny gives a total of +296 damage, to a final 1466 damage shot
   

At some point, I might upload a quick gameplay preview video of the build. But in the meantime, I can assure you this build can carry Auric Maelstrom with ease -which is my reference point for a good build. Havoc 30+ is a bit more intense, and due to ammo pick-up reductions you lose some shotgun action, but it is still viable, although it requires a sweatier approach than say, a purgatus + venting shriek build.