This was for a Ravenloft one-shot. I wrote this prior to the DMG coming out. In play, I didn't use it much, chiefly because I tend to forget about add-ons during play. It takes a while for me to build a habit and the game only lasted a set number of sessions.
House Rule: Horror
When the DM calls for a horror check, make a Wisdom saving throw. If the scene of horror represents some corruption of your established ideal, you make the save with disadvantage. If it somehow plays into your established flaw, you make the save with advantage.
If you succeed on the save, you are not horrified and may act normally. If you fail, mark 1 Horror and you are frightened (see Appendix A: Conditions) until you are removed from the scene of horror for a reasonable amount of time.
Accumulating Horror is dangerous. They are like failed death saving throws, only they cannot be removed except through powerful magic (e.g. greater restoration) or spending downtime seeking treatment in a sanitarium. Upon reaching 3 Horror, you tumble down the rabbit hole of insanity: You can't take actions, can't understand what other creatures say, can't read, and speak only in gibberish. The DM controls your movement and actions as you effectively become an NPC. At your option, you become a villain whose motivations are based on twisted interpretations of your established ideal or flaw. Otherwise, the character is effectively unable to continue adventuring until treated with powerful magic or spending downtime in a sanitarium.