Sicarian Infiltrator
 Zealot Build - Warhammer 40k: Darktide 

TL;DR This build relies very little on Shroudfield to do damage. Heavily chunk bosses with running attack weakspot backstab. Clear mixed hordes with light attack spam and fight carapace with heavy heavy light. You have martyrdom and cleave/damage/stagger talents so you don't need to use the heavy sweeps for horde clear. Always use running attack with shroudfield when fighting bosses, practice hitting their weakspot from behind. Don't empower your relic blade for light hordes, you just don't need it. Save it for mixed hordes or metal balls of death. Always go for headshots. Activating the relic blade is twice as fast if you are sprinting. Your autogun makes you immune to ranged enemies and deletes snipers/specials quickly. Don't use it to fight gunners regularly use it to close the gap against them. The las shots are for specials and snipers only, or the random gunner you don't want to waste time running towards. 

What is a Sicarian Infiltrator? They are Skitarii of the Mechanicus that specialize in infiltration and disruption. They blast deafening white noise and fry local electronics not blessed by the machine god. They were Skitarii forces ripped apart or mutilated at the end of battles, barely alive, deemed worthy enough to be remade by steel. They are armed with power weapons, stub-carbines and flechette blasters (assault rifles and smg's), and have stealth fields that make them take less damage from enemy fire. On the tabletop they are used mainly to do battlefield objectives or deepstrike into the backline to skirmish or screen against key targets, having stratagems to get them out of sticky situations. Their close equivalent (same models different headpiece), the Ruststalkers, specialize in picking out the opponent's HQ's and leaders in melee and killing them quickly before you usually would be able to (characters are usually protected until you clear the infantry around them).

In game, this build is barely alive (Martyrdom) and specializes in infiltration of the backline (Stripped Down + sprint efficiency) and eliminating key targets (Shroudfield). It's armed with a power weapon (Munitorum Mk X Relic Blade) and an assault rifle/smg lead spitting hybrid (Vraks Mk V Infantry Autogun), protected from enemy ranged fire (Stripped Down + Invigorating Revelation) and is able to do objectives or revive where others can't (Shroudfield). You can deal massive damage or oneshot bosses and key dangerous enemies while not having to worry about the horde around you, and always have a get out of jail free card in your back pocket.

Munitorum Mk X Relic Blade: Use the Mk X. For single targets use heavy heavy light, single target high rending damage. Always remember though that your running heavy will deal way more damage than your non-running ones for Shroudfield . You oneshot crushers from behind with the standing ones, but this is very important for bosses. For horde you can just spam lights tbh but light light into eternal heavies works fine too. I prefer the occasional stab in the light combo because it's super quick, helps you chain weakspot hits for cranial grounding and helps with the random elites you'll find sprinkled around in the hordes. If you have Rampage and Against the Odds you really just don't need your sword to be activated all the time. Save it for elites and specials, or when that horde gets super thick out of nowhere and you're losing positioning. 

Master-Crafted Shroudfield is a crutch. You can switch out Abolish Blasphemers for Backstabber or Time to Kill if you want to do more funny boss oneshot damage, but it just isn't worth it. Getting an empowered backstab onto the boss is more than enough; you;re already winning. Someone else can shoot the boss for 0.2 seconds to deal the damage from that talent. You need Crusher-busting power, as much as you can get, and it helps mulch those thick elite balls that are this build's biggest threat.

Curio Perks: Ability cooldown is the simplest include. Having Shroudfield up quicker is for the best. Toughness is just as important. For the final one choose gunner resist, stamina speed, or revive speed. Stamina regen speed is fine and helps you keep Stripped Down active. Revive speed is kinda unnecessary in normal gameplay as you can use Shroudfield if it's a clutch situation and revive for free every time, not to mention you already have 25% from Providence. But stacking it can help you clutch up when EVERYONE gets downed or needs rescued, so it's still a good option. Gunner damage resist is great, and don't ever let anyone tell you stacking 3 has "diminishing returns." It doesn't. Every one is a 1.25x multiplier to your effective hp against gunners. When it comes to the absolute value, the third one is actually the MOST valuable. Assuming 100 toughness with no other resistances the first one is worth 25, the second is worth 31.25 and the third is worth 39. Assuming you have Martyrdom, Hubris, Benediction and Good Balance active you take 33.867% damage to your toughness, meaning roughly triple effective hp (2.95x multiplier). This doesn't account for Invigorating Revelation, which would make it a baseline 4.2x multiplier to toughness, but it's not included to account for uptime on the other abilities. So the first gunner resist is +75 EHP, the second is +93.75 EHP and the third is +117.2 EHP. You can tank almost an entire base toughness bar (125) more gunner damage when you take the final gunner resist perk on the curio. It's very noticeable in game.

Blades of Faith are mandatory these days TBH. You should NEVER pick up nades, ever. They lack the clutch power of Stunstorm Grenade or Immolation Grenade, but are quick and reliable special killing at medium range and help keep you in a melee flow state. Whenever I use stunstorms I feel like they do literally nothing, as the stunned enemies are immediately replaced by the rest of the horde behind them. Immolation Grenade is great for throwing down at big groups to eliminate the chaff or to secure your surroundings while you revive a teammate, but scum's Chem Grenade shows just how powercrept zealot's are.