Diamantina suffered a visitation of the Tyrant Star not long ago, so you've detailed records of what transpired. According to the few survivors (a handful of Iron Warriors for the most part) it began as a sort of eclipse of the sun that didn't pass. A baleful corona of dark flame surrounding the sun. Within a few days the human population went stark, raving mad. Many suffered gross bodily mutation to go with their insanity. The insane/mutated population then proceeded to murder one another in an orgy of blood and pain, sexual deviancy, sorcerous rituals, and whatnot. In the end those who had not been killed succumbed to their strange affliction or killed themselves by leaping from the sea-cliffs by the thousands. The Iron Warriors did feel the effects of the visitation. A few broke down mentally and were eliminated by their peers. Others began to be mutated, but promptly engaged in a little field surgery to stop the taint. So marines aren't immune, only more resilient. After about a week the dark halo lifted and things went back to normal.