Deimos | Purgatus | Wrack & Ruin - WAR CRIMINAL SPARKHEAD
 Psyker Build - Warhammer 40k: Darktide 

This is my version of what is likely the most popular Psyker build, the Pyrokinetic Psyker. 

The Purgatus is basically best in slot blessings, as things like uninterrupted channeling are useless and form bad habits, suppression is already built into left click so terrifying barrage is bad, leaving only warp flurry and warp nexus as viable blessings. Crit chance is key to the build as will be explained in the Soulblaze section below in bold. 

The Deimos I run is admittedly not min-maxed, but it does the job. Precognition is my personal choice here, as it allows me to have constant uptime on crits, which  one taps all specials. It can be replaced with something like Slaughterer if you feel like you need more power to reach a breakpoint (you won't, as the H2 is the single strongest melee attack in the game). Uncanny strike/Deflector is probably what you should be looking for, but I've found a highrolled Deimos on base stats alone one shots most things in the game regardless of blessings with H2. The blessings also rely on your playstyle. I run Blazing Spirit IV as it was stuck to the 380 Deimos I was gifted, but I've grown to like the few extra stacks I see appear in melee combat, even if they don't amount to much damage in the long run, it allows you the chance of extra soulblaze stacks spreading, which equals more warp charges + more toughness regen. If you were deadset on running this build with these blessings, they would probably work better for an iliisi when it comes to actually applying Blazing Spirit, but with Purg you want to run Deimos or a super high tier dueling sword, as you've got crowds on lock with your flamer and use your melee to finish off the mutants/ragers/etc that break through the line. 

Feats are another place where people have many varying opinions, so here's mine to add to the trough.

1-3-1-2-2-2

For level 5 we go with 1, Essence Harvest is undeniably the best. As long as something is dying near you, you will be permanently regaining 30% toughness making you deceptively tanky. Warp Absorption sounds like it would be good as you're constantly killing things with your staff, but overall, the 30% regen from a warp charge x the average of at least 250 warp charges gained through a match with other feats will HEAVILY outweigh warp absorption 15% per kill, as 90% of the time you have absorption proccing, you don't even need it. 

Level 10 is what sets our build apart from most Flame Psykers, our abuse of 3, Wrack and Ruin. This feat used to be terrible. it would only proc 4 stacks of Soulblaze if you killed the elite/specialist with it. However, after being changed to hitting an elite/specialist instead, this is the bread and butter of our build that brings everything together (it still has its description wrong here). Now, BBing a crusher/mauler/bulwark from across the map not only aggros it to you, but also thins out all of the chafe that is around it. 4 stacks doesn't seem like a lot, but we will explain the damage numbers afterwards, paired with our level 25 talent, it adds up quickly and exponentially. It turns BB from a single target burst of damage that's generally underwhelming in Damnation+, to your hands planting a red barrel inside enemies heads and detonating it. I will spend 15 seconds walking to a different portion of the map with my soulblaze still ticking off who knows where getting the compliment voicelines from other classes without even knowing what I killed, the collateral damage doesn't interest me, hence the accusations of war crimes, whatever those are. 

It takes 3 BBs to kill a Bulwark/Crusher on Damnation, which means that everything in a 4 meter radius to it has taken 12 stacks of Soulblaze that is ticking away. A Crusher in a horde with 30 enemies around it (low end) being killed by you has just applied 360 stacks of soulblaze without taking your Purgatus out. WIth this, even put head to head with a flamer Zealot, you will outplace him on the scoreboard by the end of 99% of runs.

• Reapers die to the burns after 2 BB at 3 warp stacks and 15 damage ticks without other bonuses.

Level 15 Psychic Communion is BIS and basically the only level 15 you should take in any psyker build, also synergizes with level 5's Essence harvest.

Level 20 Kinetic shield helps deal with shooters, Kinetic deflection is only used in Deflector Force Sword builds, mind in motion is just bad, you should be quelling while dodging/sliding.

Level 25 Wildfire is key in hyperscaling our damage, it's effect being wrack and ruin on every unit that dies with a stack of soul blaze on it (you or another psykers). Infinite spread that can clear not rooms, but entire areas of the map with a high enough roll on a Purgatus staff, with my rolls, I could probably solo run a Malice+ using only BB and Melee.

Level 30 Ascending Blaze is a no brainer, all soulblaze kills have a 10% chance of giving you a warp charge, outputs damage, increases your toughness regen with 5, etc.

Curios are self explanatory, damage resist against specialists you hate the most, +30% revive speed ally has been clutch for me more often than not!

SOULBLAZE DAMAGE

Taken from 
The Psyker Atheneum

By Pygex https://steamcommunity.com/sharedfiles/filedetails/?id=2917274959

• Soulblaze lasts for 11 ticks, approximately 8.25 seconds. The tick interval is 0.75 seconds.
• Soulblaze does damage monstrosities and bosses even though the blue fire visual effect may not always be visible.
• When expired, Soulblaze starts to wear off one stack at the time, each lower tier ticking for 1 tick.
• Damage scaling is using the smoothstep (S1) function, therefore, the damage has approximately exponential scaling with low amounts of stacks, approximately linear scaling with 11 or more stacks and then approximately logarithmic scaling with 21 or more stacks.
• Soulblaze damage ticks take the damage modifiers (perks and blessings) from the currently active weapon and buffs, not from the buffs at the time of application.
• Soulblaze damage is always calculated against the body armour, except for Reapers which count as Unyielding.
• Critical hits apply double stacks with Purgatus, but not with Blazing Spirit on FS, Void and Trauma.
• The maximum amount of Soulblaze stacks on a target is 31.
• Soulblaze stacks are shared across multiple players. The player that makes the first application is used for calculating the damage buffs as long as Soulblaze is kept alive on the target.
• Soulblaze damage scales multiplicatively with the 10% from Kinetic Presence (Aura) and multiplicatively with the 25% from Cerebral Lacerations.
• Soulblaze damage scales multiplicatively with the following perks: +melee damage %, +ranged damage %, +armour type damage %.
• Purgatus staves can currently apply 1 stack more than what their burn % roll gives, but only if the last application is a critical hit. However, they cannot upkeep the extra stack.
• Weapons cannot increase nor upkeep the amount of stacks on the target beyond their respective caps.
• WR and the AB do not have a cap for the amount of stacks they can apply other than the maximum stacks of 31.

Table 4: Tick base damage per stack for Soulblaze
 

Stacks 1 to 5

1 - 17.41

Stacks 6 to 10

24.47 - 61.26

Stacks 11 to 15

72.09 - 118.95

Stacks 16 to 20

131.05 - 177.91

Stacks 21 to 25

188.74 - 225.53

Stacks 26 to 30

232.59 - 249.24

Stack 31

250.00