Havoc 40 knife Zealot martyrdom
 Zealot Build - Warhammer 40k: Darktide 

Gear

• Knife: Uncanny Strike is still very good even after the nerfs. If you don't want to run uncanny strike, Precognition and Mercy Killer are also good. Riposte is overkill since you already have a lot of crit chance. Flesh Tearer is okay at best, you want to kill enemies as fast as possible, not bleed them out.
• Flamer: use to clear hordes and apply brittleness to large groups of armoured enemies, throw knife when swapping to flamer to cancel animation.
• Curios: 3 wounds or 2 wounds+1 toughness, 3x revive speed is extremely strong.

Talents

• Some talents can be changed, but it doesn't make much of a difference. The only talent I'd suggest swapping for is Enemies Within, Enemies Without (remove Backstabber and pick Disdain to path to it) if you're not confident enough in your survivability. Zealot's toughness regen is indeed very conditional; if your chorus isn't up, you need to kill enemies or dodge (Second Wind) to regen your toughness, which can sometimes be awkward, this is why playing with a class that can regen your toughness is very nice (psyker through bubble, vet through voice of command and talents, hive scum through stimm crate). Enemies Within, Enemies Without allows you to get some decent passive regen.
• If you have a 2nd zealot in the team already using Beacon of Purity, you can swap to Benediction, otherwise I'd recommend to always use Beacon of Purity, even if there is no blight havoc modifier active.

Tips and tricks

• Your goal is to stay alive, chorus guarantees revives + you have crazy revive speed so the best tip is to play defensively, it's not your job to deal damage.
• Channel (5+ pulses, 7 max) your chorus for toughness + dmg boost / you can vault over objects while channeling chorus or move faster by dashing sideways / try not to overlap with Voice of Command yellow toughness if possible.
• Use chorus to help teammates grabbed by mutants, they can deal crazy damage on high havoc.
• Use med station if you have 5-6 corrupted wounds, otherwise leave it for teammates.
• Save your flamer ammo as much as possible, it's incredibly strong for dealing with hordes. If you have a vet with ammo regen in your team you can abuse the flamer as much as possible, it's just insanely overpowered. You can also use the flamer to damage bosses if you don't want to risk getting hit (especially against beast of nurgle).
• For twins, use chorus when bubble breaks to stagger and kill the melee twin before next bubble, remember to use the push attack to get attack speed from Punish Impiety to destroy bubble faster.
• For bosses, use chorus to push them into a corner or into your team so they're easier to hit, for beast of nurgle, chorus does nothing. If a teammate is grabbed by a chaos spawn, they'll get eaten until you cancel it with chorus (or a teammates cancels it with another ability); if your chorus isn't up, run up to the boss and the chaos spawn will initate a 3 attacks combo and throw your ally away from it.
• Gunners/reapers are BY FAR your worst enemy, pray your teammates deal with them, you can also suppress them with flamer if needed or with chorus - if they have the pink havoc modifier, or final toll (the red modifier on low health), or cranial corruption, they're basically immune to stagger and suppression so you can't do anything against them.
• If you're running Blades of Faith it's often your job to kill specialists (especially the ones that spawn behind your team) since you can oneshot them. They also deal good damage against unarmoured enemies like reapers/dreg ragers/mutants. They can also be used to quickly deal damage to regen health through Holy Revenant.
• Team comps: the usual team is Psyker - Ogryn - Hive scum/Vet - Zealot. Psyker and Hive scum are incredibly strong, you can also bring both a vet and a hive scum and ditch the ogryn, or swap the ogryn for an arbitrator but arbitrator unfortunately doesn't bring anything unique. Since everything kills you extremely fast in havoc 40, you'd think having a very defensive team is the best, but having more damage is actually the best, which is why psyker and hive scum are so strong while arbitrator just lags behind.