Adora's '100% Crit' Blazing Invis Knife
 Zealot Build - Warhammer 40k: Darktide 

It took me a bit to long to realize that Combat Knife can hit 100% crit with specifically Zealot. Turns out, Scourge (30%), Blazing Piety (25%), Base character crit chance (5%), Riposte (20%), Combat Blade innate crit bonus (15%), and a little melee crit chance perk (5%), all sums up to exactly 100%. 

This lets you spam whatever ability you want with infinite Invocation of Death uptime (along with all your other crit effects like Enduring Faith), and this is the build I put together for Shroudfield. Given how much you can spam it, you are constantly gaining toughness from Invigorating Revelations, so I don't run Second Wind. If you wanted to drop Duellist for it, you could, but I don't feel it's needed.

Flammer is meant for baking bosses and if the wave gets to thick, since you have such a small weapon with a knife. Can use invis for it's long reload time, throw a knife to pull it out fast. Is quite effective in a pinch, though the point of the build is to spam infinite crits with your knife.