Keep everything as it is but add a sanity score that every player character possesses. Their sanity is equal to d6+6.
Terrifying situations, interaction with outsider entities, psychic attacks and so on can drain your sanity. When exposed to this potential drain you test your sanity by rolling 2d6 looking to roll under your current score. If you fail you lose sanity according to the source.
Dead body - 1
Brutally butchered dead body - d3
Ghost - d3+1
Some kind of body horror thing - d6
Catroika Himself - 6d6
Exposure to cosmic terror drains even the most fortified mind. Some creatures or situations will have an automatic sanity drain. You express it as [automatic drain]/[drain if you fail a sanity test]. To repeat the above examples:
Dead body - 0/1
Brutally butchered dead body - 0/d3
Ghost - 1/d3+1
Some kind of body horror thing - 1/d6
Cutroika Himself - 6/6d6
So like Call of Cthulhu, right?
Running Out of Sanity
When you hit negative sanity and every time you lose any sanity thereafter roll d66:
11 - Fainting fit. Unconscious for d6 minutes.
12 - Trembling. Until you remove yourself from the situation you tremble uncontrollably. -2 to all rolls.
13 - Panic. Test your luck every time you gain initiative or wish to take some kind of action. If you fail you flee from of the presence of the sanity drain.
14 - Dumbstruck by the uncaring cosmos. You drop what you're holding and just stand there staring for d6 initiative rounds. Can be led but won't defend themselves.
15 - Get them off me! Overwhelmed by the feeling of crawling insects you tear at your skin and pull at your hair. Take no action until you successfully test your luck, while losing 1 stamina per turn due to your scratching.
16 - Unburden yourself. You strip off your clothes and possessions and then sit in a corner hugging your knees or knee analogue. Takes one round per 3 burdensome items carried to fully disrobe, starting from the top of the list.
21 - Wild eyed. People meeting you notice something a bit off. You now have a case of the crazy eyes. People will also tend to remember seeing you, making blending in to crowds awkward.
22 - Tongues. You babble uncontrollably for d6 days. You can reduce your volume but can not stay completely quiet. If you roll a 6 it is permanent.
23 - Mute. You suffer acute aphasia, unable to speak for d6 days. If you roll a 6 it is permanent.
24 - Incomprehension. Words on the page no longer make sense, they slip and slide to confound you. Lasts for d6 days. If you roll a 6 it is permanent.
25 - Trichotillomania. From this day on you nervously pull out your hair. You do it out of sight or at least infrequently, but your patchy baldness stands as evidence.
26 - Hyperventilation. You skip your next turn due to this nervous attack, but are otherwise ok.
31 - Dyskinethsia. Once per game the GM may ask for you to test your luck or perform an involuntary spastic motion.
32 - Akathisia. You constantly fidget about, twiddling, hopping from foot to foot, you can't stay still. Otherwise fine.
33 - Mental callouses. You lose one permanent sanity point and then return to your new maximum sanity score.
34 - Steeled. Now you've seen everything. Gain one permanent sanity.
35 - Cosmic insight. Gain one spell at rank 1 from the random spell list.
36 - Berserk. Gain d6 skill but you must attack someone every initiative. Lasts d6 turns.
41 - Nervous vomiting. Spend a round evacuating your guts, lose 1 stamina. Otherwise fine.
42 - You are strangely ok with this
43 - Gnashing of teeth. Lose d6 teeth.
44 - Kleptomania. The GM will arbitrarily assign items from your environment to your inventory. This might be from other players, or it might be evil ancient artefacts.
45 - Pyromania. When around fire you must test your luck every hour or burn something significant.
46 - Compulsive diet. You secretly eat some inedible substance as a coping mechanism. This costs 1 stamina per day and lasts for d6 months. On a 6 it is permanent.
51 - Hysterical laughter. You laugh in the face of madness! And laugh and laugh and laugh. This continues for d6 hours. If you roll a 6 you die from laughter right there on the floor.
52 - Auditory hallucinations. You hear a loud ringing, screaming, whispering, crunching or other unpleasant sound which distracts you. -1 to all rolls for d6 days, if you roll a 6 it is permanent.
53 - Delirium. You can move and talk but no longer know where you are or what you're supposed to be doing. -4 to all rolls due to confusion and incoherence. Lasts d6 days, if you roll a 6 it is permanent.
54 - Mad flight. If there is a window you will smash through it, if not you bash your head against the wall or a floor until it passes. Damage as falling or as club, lasts d6 turns.
55 - Auto mutilation. You start harming yourself with whatever you're holding. Deal damage to yourself every turn until you successfully test you luck. May defend yourself but do nothing else.
56 - Never the same again. No outward signs, but you lose 1 point of permanent sanity because of the experience.
61 - Screaming forever. You never stop screaming, ever. After d6 days you lose the ability to speak at all and it goes quiet.
62 - Hysterical blindness. For d6 turns you are completely blind. You recover fully when it ends.
63 - You scream like an animal with its leg in a vice, raw and disturbing. Everyone else must test their sanity or lose 0/d6. You stop screaming when removed from the source of terror. People test only once.
64 - It cannot be unseen, but you try. You attempt to gouge your eyes out. Test your luck. If you fail you succeed and suffer 3d6 stamina loss.
65 - Fugue state. You gain no experience checks from any rolls made during this game (including prior to the event) because you don't remember it. You wake up at the beginning of the next game feeling very confused.
66 - Scared to death. Make a new character
Regaining Sanity and fixing your donkey brain
If you spend a night in safety and comfort you return to maximum sanity. Dungeons and the wilderness never count.
If you have a permanent madness then you're shit out of luck. Put out feelers for a Width Doctor or Oneiromancer and hope they can help.