Assassin / Shroudfield Zealot (Current)
 Zealot Build - Warhammer 40k: Darktide 

This is my current favorite Zealot build. It may actually just be my favorite build in the game, it's too damn fun.

Short Link: https://youtube.com/shorts/GSJxondi2O4?feature=share

Pros: Fastest build in the game, high burst damage, great AoE, pretty tanky.

Cons: Your only special sniping tool is a throwing knife and you can't return fire on gunners from really far away.

Weapons

You can run any weapon you want with this build, Shroudfield is very flexible and this build gives you plenty of attack speed - but I still recommend running any weapon that gives you Uncanny Strike because it allows your Flamer to melt Crushers (who have more HP than ever before). 

For your ranged weapon run the Flamer with Blaze Away and Overpressure to boost the damage of your burns. You can make a case for running the Boltgun, Bolt Pistol, or Revolver but all of that boils down to personal preference. 

Curios

For Curios I run a balanced setup with 2 toughness and 1 stamina. Grab Ability Cooldown, Stamina Regeneration, Gunner Damage Resistance, Sprint Efficiency, and Block Efficiency. You're going to be using your ability whenever it's up so you really want as much cooldown reduction as you can get and stacking stamina benefits makes surviving much easier. Stacking toughness curios allows you to mitigate more damage in the long run.

Talents

Start off with Backstabber because it's the obvious first choice for an "Assassin" build like this and bridge into Second Wind for the extra toughness regeneration and Duellist and Purge the Unclean for the extra damage.

Our Blitz is Blades of Faith because it's the only way we can snipe specials when using the Flamer.

Until Death, Holy Revenant, and Shield of Contempt give you access to some seriously broken survivability tools when they're in the right hands. These make the best of players unkillable and add a lot of value to the experience for players who are new to the game. 

Sustained Assault, Desperation, and Faithful Frenzy provide good damage increasing value while building into our Aura, Benediction, which is still one of the best auras in the game because toughness damage reduction is so valuable.

For our ability we take Shroudfield and all of its modifiers. This allows us to get extra damage resistance, backstab damage, increase our stealth duration, and reduce our cooldown when we land backstab kills from stealth. It's a good set of supporting modifiers for a very good ability.

Our first Keystone is going to be Pious Cut-Throat. Technically Martyr's Purpose provides more value over the course of a mission, but we're not going to build into Martyrdom because Inexorable Judgement is actually really good now.

On the left side of the tree take the crit package: Scourge, Enduring Faith, and Blinded by Blood. Great damage and toughness damage reduction increases.

On our way to the final Keystone take Relentless Fervor to move FAST and then build into Inexorable Judgement with all of its supporting nodes. This will allow you to get to 20 stacks of Momentum really easily, which will give you 8% toughness regeneration per second, 20% increased melee and ranged attack speed (huge for flamer), and a 20% increase to all damage - including damage over time effects.