Momentum, Chorus: Support/Horde clear Zealot
 Zealot Build - Warhammer 40k: Darktide 

Intro:
This build focuses on Mobility with Good stamina regen, momentum and increased movement speed.
It will also work well in higher difficulty as a team player build with some excellent Wave clear and stagger. 

Chorus of spiritual fortitude will allow your team to recover and reposition in case things go south and will allow easy cleanups. 

Pros:
Super mobile, you will almost always have a possibility to reposition and you will be able to close the gap incredibly quickly.
Massive cleave damage and stagger will allow to easily dispatch Waves of enemies you will be able to hard stagger most hostiles, every single hit except for maulers, crushers and mutant ragers.
Unending toughness, as long as you have mobility or targets to kill you will never go down and never take damage, no matter how reckless you are.

Cons: 
Rather low boss damage, A Revd. Heavy strike will do between 3.5k body to 6.5k  weak spot hit.

Too fast for your teammates. you will have to be careful not to get stranded in the open with your teammates miles behind.
Very fragile to trappers and dogs, luckily they aren't too hard to evade.

Weapons:
Melee:
After testing I found viable options.
The best: Tigrus Mk XV Heavy Eviscerator.  
This variant has the easiest combos to learn, The base weapon damage is extremely good, with large and powerful cleave and it has Massive range for a melee weapon. 
Worst stat: Shredder, Do not get fooled, Mobility is a great stat, More mobility = more speed and more momentum, more momentum = more damage, attack speed and sustain.

Perks: 
Best: 25% damage to Maniacs, flak or unyielding, unarmored.
Good: Critical Hit, Weak spot damage, Sprint efficiency, Block, Stamina
Bad: Damage to carapace, you will not attack crushers with this weapon. and you will priorize hitting maulers in the body.

Blessings: 
Best: Savage Sweep, Rampage, Momentum
Good: Wrath, Shred, Rev it up.
Bad: all other blessings are sub-optimal since we do not play with crit, and the gain of crit, is also very minor.

Other Options:
Cadia MK XIIIG Chain swords
Similar to the eviscerator but much quicker, this sword will shine against smaller waves or vs medium size enemies, Sadly it lacks stagger and has a very small cleave radius, At lower levels this sword was a great performer, but it doesn't scale as well in heresy/damnation.
Chain Axe:
The chain axe has solid single target damage but it suffers the same way the chain sword does with hordes.
Heavy Sword 
The heavy swords are viable early on, but as you reach higher difficulty and face more and more armored enemies its value greatly diminishes
 

Ranged:
The Best: Godwyn-Branx Bolt pistol.

This weapon is the perfect fit for this build, it has some of the best stopping power out there, Massive damage, Quick reload (2.5s or less) Great stagger, even if you miss your target. and run and gun is a god tier blessing! 

Perks: 

Best: Damage to Flak, Carapace, Maniac, Reload speed

Good: Damage to weak spot, Sprint efficiency, Stamina, crit chance

Bad:  anything else really, even the "good" ones are Ok at best

Blessings:
God Tier: Run and gun. ABSOLUTE MUST HAVE, this will allow you to pretty much snipe enemies with lethal accuracy while sprinting towards them, do not sleep on this, it is a true life/game saver

Great: Puncture for boss/elite damage
or Lethal proximity if you suck at aiming, it will often stagger enemies and give you time to reposition

Other options:

Shotgun, It works, but it just isn't as good as the bolt pistol and it has awful boss value.
Revolver, It works,  close competitor to the bolt gun but not quite it.
Boltgun, decent for boss killing, is extremely slow to swap to and reload, also has a very low ammo count. does not play along with the builds mobility.

Curios:
Since we are stacking a lot of toughness I recommend either 3 toughness Curios or 2 toughness and 1 stamina Curio.
Perks:
Best: Toughness, Toughness Regeneration speed, Stamina Regen,
Good: Sprint efficiency, Block efficiency, Damage resistance

Bad: All others

Talents:
Chorus of spiritual fortitude/Holy cause:

A solid life saver skill, the 30% damage reduction really helps when it counts the most, this perk also staggers ALL enemies, Crushers, Ragers, Plague Ogryns, Reapers, dogs, Beasts of Nurgle, ETC, allowing you to flee, reposition, save your team or hard stagger a boss so your team can go HAM at it. 

Immolation Grenade:

I am not a fan of the Sunstorm grenades, and the blades of faith tree sadly do not work well with the crit less build.
The immolation grenades offer a decent crowd control ability, they work wonders with choke points and they do up to 7000 damage to a boss if you can keep him in it (Ground fire damage does not stack with itself so make sure to delay them)

Aura:

Benediction:
The base aura for zealot, it is very strong, with more melee damage and move speed on the way to it, along with Holy revenant which is very good for learning players. You can easily live without it once you are familiar enough with the game mechanics though. For me holy revenant only procs when I'm completely pinned down and I cannot use the healing part of the talent, usually end up dying anyway as soon as it ends, it does save once in a while when you fail to dodge a sniper or a crusher surprises you on a corner but that is anecdotal and it has very poor value.

Sadly Loner does not add much interesting value to the build and reduces your group usefulness by a lot.
Beacon of purity is generally bad, unless you struggle with corruption, and Med kits are rather rare in the game and the nodes to reach it are of very low value, 

Momentum, Retributor stance, and inebriate poise:
Momentum is a key part of the build, as long as we are moving we gain momentum (stacks rapidly) upon spending it (hitting an enemy with ranged or melee attacks) we gain attack speed, damage and most importantly we regen 30% of our toughness, for the next 6 seconds we can stack more momentum stacks to expand as more toughness regen, I usually manage to easily generate 6-9 stacks in these 6 seconds while wave clearing, in a scenario where your team goes down this will allow you to save the day or drastically increase your survival rate with boss aggro.

Thy Wrath be swift:
Incredibly good talent, the stun resistance is a life saver often and the movement speed buff on hit is an ok bonus. 

Sustained assault: 20 % near 100% uptime melee damage
Swift Certainty:  More speed is good :) and the dodge part of the talent will save you a lot of damage taken while moving 

 

Replaceable talents:

Pious Cut-Throat:
For every enemy we kill from backstab hits, we lower our Chorus CD by 12 seconds, when clearing waves and with how mobile we are, this has a decent use, it can be skipped though 

Faithful Frenzy:  While 10% attack speed is great, and it opens the way to a 10% extra Health talent, 

Stamina point:

The extra stamina point is pretty much mandatory if you are taking 3 toughness curios, but if you are planning on taking 1 stamina curio you can spend that point where you desire.