Gandalf: Melee Wizard (Updated for Bound by Duty)
 Psyker Build - Warhammer 40k: Darktide 

I always loved that Gandalf was a wizard who fought in the thick of it with a big ole sword and ever since I started playing Psyker I wanted a big ole sword of my own. Now with a refreshed talent tree (Even better with Bound by Duty) and Covenant Force Swords we finally have truly frontline viable melee Psyker that can go toe to toe with the big boys and chop them good and hard.

TLDR: The goal of this build is to maximize your time and damage in melee while not throwing away the utility a Psyker can bring to the team. Pop Scrier's Gaze as much as possible for a damage boost vs elites or to farm stacks from a horde. Learn your combos, dodge and chop. Whip out your staff to soften up particularly big groups of elites or to provide CC in an oh shit moment. Use Brain Rupture to pick off dangerous shooters from cover and auto-pop specialists with your sword's force wave.

General Build Theory:

This is designed to  be a Psyker that you can basically play like a crit Zealot. You are running around like a warp-fueled murder-hobo killing whoever the Beloved tells you to and then raging out into every pack of elites or hordes to build maximum stacks of damage buffs. Your crit chance will climb above 60% during Scrier's Gaze meaning you are practically lighting everything on fire with Soulblaze triggering further crit chance and toughness regen. Using the movement speed buffs from Mettle and Disrupt Destiny you will be extremely maneuverable and able to chain from kill to kill with ease. Probably the most important revision to this build has been the addition of the new perk, Warp Ghost. This enables you to nearly permanently run at high peril maximizing the buffs from Warp Splitting, One with the Warp, and Warp Rider, but it did make Assail much less practical as a sidearm/utility ability. Thus we have returned to Brain Rupture for sniping at extreme range or picking off gunners while dodging into cover because it requires less peril to do significant damage (a single Brain Rupture can kill most specialists, a single charged Assail only sometimes). Kinetic Flayer will provide an occasional damage spike into tougher enemies, but is mainly useful for instant kills on dangerous specials. Your staff is retained (instead of the gun common on many melee Psykers) because it allows you to provide extreme CC when the director flips out and it's not safe to just chop and dodge.

Tips:

• Use heavy attacks as much as possible to one shot enemies. Hold attacks and slide to chain these high damage hacks and stabs.
• Light attack spam is entirely viable into hordes, but mixing in the horizontal heavy slash helps.
• Spam Scrier's Gaze with reckless abandon. You aren't going to "waste" this ability even if you only secure a few kills. It still makes you stronger, more safe, and with the reduced cooldown from Psykinetic's Aura it will be right back.
• Use your sword's special ability, even if it isn't fully charged, to stagger enemies and AUTOMATICALLY KILL dangerous specials in mixed hordes. You can always use this ability to stagger enemies and give yourself some space and if Kinetic Flayer is up you can automatically kill the Scab Trapper or Pox Hound in the mixed horde that is about to disable you.
• Utilize the quickdraw attack for the sword by attacking right after switching to melee. You can stagger or knockdown an enemy with your staff and then quickly switch to the sword for an off-the-shoulder attack animation that is very quick.
• Either version of the sword is viable. The Covenant Mk VI Blaze Force Greatsword has better DPS and a super easy single target chain, but the Covenant Mk VIII Blaze Force Greatsword has really useful sweeps and a much stronger push attack stab that can one shot lots of enemies.

Build variations:

I am still very much getting a feel for the refreshed melee Psyker after the Bound by Duty update and I doubt this is my final version of this build. If you want to maximize your glass cannon status you could drop Empathic Evasion for Vulnerable Minds, at the risk of being more vulnerable to gunners (this is the most likely change I will make). If you preferred the classic ramp up ramp down peril ride from the previous patch you could drop Warp Ghost and switch back to Assail freeing up two points that you could invest in Vulnerable Minds for more damage and Just a Dream for tankiness. By Crack of Bone can extend your Scrier's Gaze runs, but at the expense of being weaker in cooldown because of reduced Peril.