Sister of Battle | Chorus | Beacon of Purity | crit build
 Zealot Build - Warhammer 40k: Darktide 

Edits based on feedback: 

1. added Enduring Faith and swapped Holy Cause for Ecclesiarch's Call. Toughness DR is now provided through the main feature of the build, allowing pushing for damage amplification. 
2. Removed Until Death and Holy Revenant in favor of spending points in more general nodes like Second Wind, Anoint in Blood and Purge the Unclean. The amount of toughness regeneration and Chorus of Spiritual Fortitude will hopefully decrease the need for the Until Death safety net. With Anoint in Blood and Purge the Unclean the Bolter should have an easier time deleting bosses.
3. Swapped some perks around, no longer opting for +10% crit damage and instead focusing on armored targets with more raw damage.

Note: Alterative options

1. Tigrus Mk XV Heavy Evisceratorfor melee. Here you also bring the 5% increased crit chance and 10% increased crit damage with the Rampage and Shredblessings. Shred gives 20% additional crit chance on chained hit, carrying this build all the way up to 65-85% crit chance when in the heat of battle. The tradeoff is that you don't have the oomph that the Munitorum Mk X Relic Blade has and your mixed horde clear is not as strong though still viable. Preference based on wanting guaranteed crits or better raw damage. (And, ofcourse, your preferred power fantasy)
2. Artemia Mk III Purgation Flamer for secondary. The Flamer is anti-everything but lacks range. While very iconic for an Adeptus Sororitas, a Bolter is part of the trinity as much as the flamer and the melta. Choice is purely made for having ranged options (that can also deal with pox-armor and carapace) rather than needing to rely on your teammates to deal with far-out targets you can't reach.  
3. Ecclesiarch's Call over Holy Cause. Holy Cause is picked here for the sake of increasing Chorus of Spiritual Fortitude's safety net usage rather than an amplifier for damage. Ecclesiarch's Call is probably better in the long run but I've found to enjoy the additional damage resistance in tough situations. 

High level support crit snowball built.

How does our crit work?

1. Base Zealot crit chance: 5%
2. Scourge from 10 to 30% crit chance optimally
3. Munitorum Mk X Relic Blade 5% crit chance perk
4. Blazing Piety 25% crit chance with Righteous Warrior 

Cumulative: 45-65% crit chance every hit depending on Scourge . 

Every critical hit applies bleed (feeds into Scourge) and every critical hit activates Invocation of Death for faster Chorus of Spiritual Fortitude recharge, potentially bringing it down to 20 seconds.

Every crit grants Enduring Faith which gives 50% damage resistance to everything.

Critical strike kills also refresh Heatsink on your Munitorum Mk X Relic Blade allowing you to stay charged up longer.

Ideally, through infinitely looping charged states and crits you will find a huge damage output against any enemy type and when things get hot you always have the Chorus of Spiritual Fortitude safety net.

When facing a beefier target like monstrosities or carapace armor, swap to your Locke Mk IIb Spearhead Boltgun and start blasting. Emptying your mag will also activate The Emperor's Bullet for more impact strength (stagger) and melee weapon attack speed. 

When facing anything else in a mixed horde, use the wide sweeps of the Munitorum Mk X Relic Blade to activate Rampage and Punishment for 36% increased damage and 25% increased stagger. At this point most things that come in range will get a mouthfull of energy and die or fall on their ass for a second serving. Killing with heavy attacks regenerates 10% toughness through Vicious Offering.

Other talents of note:

Blitz: Stunstorm Grenade for making space in tough situations when Chorus of Spiritual Fortitude is not available. It also allows YOU to participate more actively as you aren't standing still emoting. This is handy to get the snowball rolling. Immolation Grenade also suffices and might fit the fantasy better, but my preference goes to stunstorm for the huge stun.

Aura: Beacon of Purity for lowering corruption pressure during Havoc. If this doesn't work that well then respec to Benediction but you likely won't need it thanks to Shield of Contempt and Holy Cause 

Safety net: Until Death which will make sure you survive when taking a big load of damage unexpectedly. Holy Revenant could be a very useful option here, and if it feels required then swap out Faithful Frenzy . This is preference in my case.

The biggest downside of this build is that it's a snowball build. When your Blazing Piety has ran out and you're ambushed you'll need to

1. Make sure 25 things around you die to get Blazing Piety's 25% crit chance increase
2. Get enough enemies bleeding and keep hitting them to get 30% crit chance increase from Scourge  
3. Continuously hit 3+ enemies to keep up Rampage and Punishment 

When these 3 prerequisites are met, and only then, the build is at full capacity. Almost guaranteed crits, everything bleeds, 36% increased damage on the Munitorum Mk X Relic Blade and a 20 second cooldown on Chorus of Spiritual Fortitude 

After the use of Chorus of Spiritual Fortitude and performing all 5 pulses both Scourge and Rampage will likely have ran out. This is a tradeoff but quickly reactivated due to Blazing Pietystaying active thanks to your teammates continuing to kill heretics.