Event explained

1. Zryachi spawns on the map, walks around, is friendly, drops Vodka, Moonshine if you are lucky, if you come close to him, if he is not killed, raid is normal (bosses spawn as per usual) ((he is always there)) (((does not appear on Factory or Labs))) ((((You can extract at any extract normally))))
2. Kill Zryachi (if you kill him, all extracts open for you, but only you) ((He dropped his guns and a rig Khamelion, mags, nade and IFAK, additional sources say he drops his mask but it did not drop for me))
3. Sky goes dark, countdown begins, blue beam spawns out of the sky and cultists start to surround the area where there is a ritualist guy who is waving their hands and is doing the ritual. (Only one extract remains open) ((It takes 30 seconds to extract))

4a) Kill the cultists + the priest, ritual stops, raid is back to normal (Still takes 30 seconds to extract) ((They are all equipped with silenced MPXs with Quakemakers, pistols with BT rounds, AVS or Alpha Rigs, their pockets have goodies – Marked keys, keycards (((My squadmate found Red))) so you always turn massive profit))

5a) Extract with shinies.

OR

4b) Don’t kill the cultists, let the priest finish the ritual, sky goes dark forever and scav hordes spawn together with ghastly Zryachi that has 6666 HP on each limb. Your gun auto misfires, so the only way to actually fire is nades and Pshz Pshz (or how you spell it) since the gun is physically incapable of jamming + there is seemingly an endurance debuff.

5b) Stay in raid, kill stuff with Nades and Pshz, maybe even the cultists, if you are a mad man, Zryachi chases you with a melee weapon and tries to shank you, moves at walking speed, but if you stray too far, he will whip out the RSASS and dome you, so keep him at arms lenght and if you got enough willpower and teammates, you can kill him with his enormous HP pool (He drops unlootable variant of his headgear and mask with a cultist knife and two mags for RSASS)

6b) Extract.

Airwingmarine made a video about this whole thing, if you want to, watch it here, nice and summarized with additional details.