STARKILLER
 Psyker Build - Warhammer 40k: Darktide 

The most important thing is maintaining your lightning uptime as much as possible. This build is entirely focused on crowd control for high difficulty missions with crazy enemy density. You need to play the game as if you're incapable of damaging enemies and move like a ninja. Your teammates are your damage source and you are their permanently active stun grenade. 

The main perk cycle you should be looking for is getting activations of Empowered Psionics from special kills and holding your chain lighting for as long as possible. This build has an absolutely crazy amount of stalling power and can save your teammates by entirely preventing enemy damage/movement for 5-15 seconds at a time, creating space for revives & dmg. This is one of the few Psyker builds you need to be using Quell ALOT to ensure you always have your lightning at the ready. Any small moment of downtime you should use as an opportunity to reset your Peril.  In bad situations where you & your team are surrounded the ideal play is to have Emp Psi available, use it to hold as many enemies immediately surrounding as possible, then when your peril hits 90-95% activate Scrier's to reduce it and buy your team even more time. Because Scrier's bumps your Quell speed you can then force push everything in the vicinity when you're back to 95% and it should buy you enough time to reset your peril to 0-25 before you need to stun-lock everything again.

Another good strategy when surrounded is to spam activate your lightning by only holding it for a few seconds at a time and knock down as many enemies as you can, as this creates room for you to sprint-dodge over the top of them to reposition.

Weapons are just recommendations for what I've found works well, the heavy lasgun is great for picking off far away special enemies like snipers, trappers & poxbursters which will cause problems for your team, a triple/quad tap headshot has just the right amount of damage. Your sword is, ironically, your shield. In situations where there are lots of ranged enemies too far away to stun and no cover Deflector will save your life, you just need to pair it with smart cover-to-cover sprinting and dodges to throw off enemy aim. 

The reason there is so much stamina, regen & sprint eff built into the perks is you need to be dodging REALLY well. Be very aware of your surroundings, where your teammates and the most dangerous enemies are. The biggest threat to you is monstrosities & bosses that are immune to your stun, but Crushers & Maulers can be problematic  too if you let them get too close and corner you. Fair warning, it takes the right teammates to properly capitalise on this build's strengths, on Damnation & higher I haven't had any issues because more experienced players will naturally see the benefit of the lightning and target who you stun; less experienced ones however will often hang you out to dry by leaving you alone in front of 5-60 enemies and at that point you just run like hell to whichever Ogryn or Abitrator on your team will happily slash away at anything within six feet.

The only perk swaps I recommend are removing Penetration of the Soul & its adjacent +5 Toughness in exchange for a DR perk like One With The Warp or Immaterial Focus + Enfeeble. This will let you be a bit tankier if you struggle to stay alive while getting used to the playstyle. Happy star-killing.