Someone on Reddit asked for how to integrate Call of Cthulhu insanity checks into D&D, and I responded with some rules I’d cooked up. This is slightly changed from that.

Depending on the nature and power of the situation, shocking, terrifying, and horrifying encounters may cause a character to have serious reaction.

• Shock: Save to stop yourself from crying out or else suffer shock (cry out and be visibly shaken for 1d4 minutes). If you roll a natural 1, you panic (run away for 1 minute).
• Terror: Save to suffer shock only or else suffer shock and panic. Lose 1 point of wisdom. If you roll a natural 1, you suffer temporary madness.
• Horror: Save for 1 point of wisdom loss and panic. Otherwise, lose 1d4 points of wisdom permanently and suffer temporary madness. If you roll a natural 1, you acquire a form of permanent madness.

Temporary Madness

1d20	Effect
1	You suffer shock (cry out and be visibly shaken for 1d4 minutes) and also trauma that produces a permanent condition. See Permanent Madness, below.
2-4	Uncontrolled violence toward all for 1d4 rounds, then shock.
6-7	You suffer paralysis for 1d4 minutes as well as shock.
8-10	You suffer blindness for 1d4 minutes as well as shock.
11-13	You suffer shock and become terrified of others for 1d4 minutes. Your hair grows out white from now on.
14-16	You suffer shock and at the earliest opportunity wander off and are found lying down, unable to remember what happened.
17-19	You suffer shock and hallucinate a comforting loved one for 1d4 minutes.
20	Panic (run away for 1 minute), then shock.

Permanent Madness

1d20	Effect
1	Phobic–You develop a phobia related to the current surroundings.
2-3	Violent–You engage in uncontrolled violence toward all for 1d4 rounds.
4-5	Reckless–You become reckless (as a martyr) and have little regard for your own safety.
6-7	Paranoid–You often imagine danger; when under stress, you don’t trust anyone.
8-10	Panicky–You lose your edge. Check for panic when under stress (start of combat, etc.).
11-13	Obsession–You become obsessed with an activity or idea.
14-16	Raving–You rant about perceived wrongs and imagined happenings.
17-19	Haunted–You get lost in reverie, reliving past horrors and seeing things for 1d4 minutes.
20	Hallucinations–You hallucinate a comforting loved one for 1d4 minutes.

If your wisdom drops to 3 or 4, you develop a type permanent madness (if you don’t already have one) and suffer an episode once per adventure in a stressful situation. At wisdom, this happens once every day. At 1, you are utterly mad. Some NPCs should have very low wisdom as a result of encountering such horrors.

Alternatively, you could say it is a loss of charisma, as it measures NPCs’ reactions to you, which tends to drop as you exhibit symptoms of madness. However, a person can’t really be both mad and wise, but they could theoretically be both mad and charismatic.

You can regain a point of wisdom/charisma lost this way by donating to a church for divine magic or by spending extra on carousing or (for wizards) on arcane tomes.