They were exposed to a horrific scene and realisation, but a thoroughly mundane one, rather than supernatural or cosmic in nature. Thus, I rolled a d2, and happily doled out psychic trauma.
- Mundane horrors - d2 to d4, based on magnitude and personal involvement. e.g. "Noooo! Soylent Green is made of people!!" = d3, while "..and that's my mother's ring in that box of it!!" = d4.
- Supernatural horrors - d4 to d8 "..but the model was a photograph from life!!" = d4, but "..and then he knew that these ghouls and wights had surely been reading Carcosa!" = d8
- Cosmic horrors - d8, 2d4, 2d6, a range from exposure to black gulfs and stygian realms to meeting Yog-Sothoth for tea in his drawing room.
When the total of insanity points equals the character's wisdom score, roll a save vs. spell (my usual go-to for mind effects.) Success reduces your insanity point total by 1d6 points. Failure reduces your insanity point total to zero, but inflicts a randomly selected insanity upon the character. I prefer random, because that reinforces the chaotic nature of the process, and many good random insanity tables can be found. I especially like the table in the old first edition Warhammer Fantasy Roleplay, but have also used the one in the first edition Dungeon Master's Guide.
Cures for insanity are generally the usual cleric spells, such as restoration, but the victim might also benefit from lengthy rest at the proper sort of institution, if available. Unless the institution provided horrors of its own, that is :)