It's a random event, you get a little message before it happens saying "The enemy are moving against each other" then it starts to spawn waves of increasingly higher ranked enemies until you get the Hallowed Knights, Wizards, Servitors, Captains etc. Was one of my fave moments on the Beta (except maybe unleashing the hive on the moon, reminded me of the flood level in H1!)
English
-
I don't think i ever saw hallowed ones. The scariest ones i can remember were ascended knights and chosen acolytes with cursed thralls.
-
Ah makes sense :)