Psyker Commando Assailant
 Psyker Build - Warhammer 40k: Darktide 

This build gives you a tool for most circumstances, but provides strong horde clearing potential.

Assail tends to land a staggering hit on pretty much all enemies but Crushers and Bulwarks, though it may take a few shards to stagger Reapers, and can stagger Bulwarks if the shard comes from any unshielded side.

This allows it to be used as a consistent utility during matches, throwing a few shards at almost any specialist/elite will provide you the time to swap to the rifle and pinpoint the target, often dropping them quickly.

This can prevent Trappers, Bombers, Tox & Flamethrowers from even firing, and can stagger Ragers and Maulers giving you a moment.

Warp Rider turns your Peril into a damage ramp, rushing hordes will let you fire assail until your peril hits 80-100, then you can swap to gun or sword for a buff. Melee kills will ramp your Slaughterer stacks, as your peril naturally lowers exchanging the damage ramp from peril to Slaughterer.

Perfect Timing and Malefic Momentum provide additional damage ramps for Critical hits and for killing a few enemies with your gun or sword, that will both strengthen assail shards and provide time for them to charge.

Generally save your Venting Shriek/Psykinetic Wrath for specific circumstances. Building 6 stacks of warp siphon is a 24% damage ramp bonus for all attacks.

The best times for it's use are to stagger a horde to save a teammate or chain over 10 assail shards on a large pack, to push your way out of a corner, or save yourself from peril implosion.

The ideal role for this build is a combination of anti-horde/elite and anti-gunner/sniper, it is not ideal dps vs bosses, but can steadily help with good weak-point aim.

You have the ability to block shots allowing you to reposition in high gunfire scenarios, and assail can stagger/clear groups of gun armed enemies well. It's aimed assail is incredibly helpful against long distance targets in cover, this includes snipers which can be taken out with the aimed assail shards, or with a gun.

The reason why Deimos Blaze is a good choice for this build is it provides a utility against Maulers, Crushers and Mutants because its charged attack will stun Maulers and Crushers, or will often 1 hit, or leave Mutants near death, if charge-struck during their passing.

Charging the Deimos sword, block-pushing, then attacking can sometimes help separate smaller enemies from the heavier target your aiming for, and with Bulwarks, sliding to the left with a charged attack ready or back-left with gun in hand tends to bypass their shield. 

Unload on most elite/specialist/ogryn/boss enemies as much as possible, this is your core damage vs any single target and is pinpoint.

Other Gun Options?

Death Spitter can generally be treated as similar to Fire Frenzy, but as an alternative Terrifying Barrage can add an additional but limited crowd control tool that can buy you time.

Viable alternative guns are Agripinaa infantry, Columnus Infantry, Vraks III (3-rnd burst with Crucian Roulette and Headhunter for the sniper biased types, this also opens Mettle as an option) or Shredder Autopistol (Powder Burn, Blaze Away or Cavalcade are all preferable)

Unyielding, Reload speed, Flak armored, Maniac or Critical damage are all options, but added Crit rate is universally helpful. Ideally : Crit & Unyielding, or Flak & Unyielding, if your willing to sacrifice damage vs bosses, Flak & Maniac.

The middle bridge between Assail/Mind in motion and the 3 abilities can be swapped around in the case you would prefer a different ability.