Parry the Psykypus
 Psyker Build - Warhammer 40k: Darktide 

This build really makes you feel safe in any situation. But how does it achieve that and what does it give you?

First of all this build has incredible survivability.
With a solid groundwork of Crit-Chance, Mettle, Quietude, Scrier's Gaze and By Crack of Bone you have a solid toughness source in any scenario and Disrupt Destiny fills any blanks.

The Devi's Claw sword with Shred helps keep the crits up and is an incredible clutch tool with Parry and Kinetic Deflection. Maulers, Maniacs, Crushers, Bosses and Demon hosts can be safely face-tanked, though you do have to be careful not to accidentally parry another attack before the Crusher actually hits you.
If you're ever stuck somewhere you can easily parry and dodge even if cornered by multiple elites. Parry gives you Peril, which you can dump with Scrier's Gaze (gaining Toughness through Quietude) if it gets dangerous and then you have time to reach 100 Peril again and then you STILL have 10 seconds with Warp Unbound where you can Block/Parry all you like or switch to Trauma staff staggers while sliding away. Btw. Parry staggers any enemy other than bosses and kills very quickly with weakspot hits, while lowering your peril at the same time. As a last Synergy since you can gain True Aim stacks with melee attacks and we always want to be going for weakspots anyway you usually have a Crit lined up and every enemy hit by the Crit-explosion triggers Mettle and Perfect Timing.

Now that's a clutch scenario, but with the Trauma Staff with both Blazing Spirit and Rending Shockwave you can usually just blast your way out of most encounters. If you have good teammates you can Stagger anything below bosses with a couple charged blasts, loosening them up and keeping your team safe. If you're alone you can easily take on 15 Crushers/Maniacs with Charged attacks and Scrier's Gaze + Warp Unbound just keeping them staggered and whitteling them down through Soulblaze, Brittleness and your attacks. For Bosses I usually go for two fully charged attacks and then light attack spam for a couple seconds to build up Soulblaze.

Last but not least in our toolkit we have Assail, making it all come together:
The playstyle into most hordes works through quick switches between your sword and assail to keep you peril at a steady high level (40-80). this not only helps thin out hordes but also helps to target nuisances or priority targets not next to you. Taking out Disrupt Destiny targets, pre stunning Trappers and Hounds before they reach you and suppressing multiple ranged units out of reach can all be done with this tool. Now since Assail usually hits weakspots it can gather True Aim stacks well and a Crit hitting 3 enemies immediately gives 3 Stacks for Mettle and Perfect Timing.

Some more synergies:

With the Empathic Evasion node, Scrier's Gaze active and the help of Shred you are pretty much invincible to ranged attacks as long as you have enemies around you. this also gives you constant high Toughness generation and high Toughness damage reduction. With Scrier's Gaze up, YOU are the Tank.
By Crack of Bone helps keep Scrier's Gaze up very long.
Assail can help gain Precognition stacks, though in my experience it's never needed.
Disrupt Destiny heavily boosts both Assail and your Melee attacks.
The constant switching between your Sword, Assail and the occasional Trauma blast keeps Malefic Momentum topped up with enough enemies around.
Parry staggers the enemy so the return attack gets an immediate boost from Skullcrusher.

What can be changed:

If you find a node to take out I would suggest going for Cruel Fortune and Perfectionism in the Disrupt Destiny line since you go for weakspots a lot and can take out marked enemies quickly with assail.
I didn't do the math for Breakpoints so if you want to change around any weapon perks or Skullcrusher be my guest.

Weaknesses:

While this build can easily suppress large amounts of gunners in a medium distance with Assail and even Ogryn gunners with the Trauma staff, being caught in the open can quickly melt your toughness.
Poxbursters are a threat to any Psyker since a few are anough to kill you outright. Assail can also push them in a direction you don't want so be careful.
While Bosses are not that scary to you per se, you do not have the highest boss damage yourself so partnering up is quite helpful. Try to get the debuffs and DoT on the boss early and then parry them while your team takes care of the rest if possible (Skullcrusher and Brittleness help out your team to seal the deal quicker as well)