So... After reading a page from the grotesque tome on the satanic alter, you must now save vs spells or roll for insanity... ah you failed... now you suffer from Equinophobia... the fear of horses...
Huh?
In most games, when you go insane, you accrue insanity points, which ultimately leads to losing your character, and/or, you roll on a phobia table to find your new fear. In either case, it's usually up to you to role-play the symptoms of your new affliction which may or may not make any sense at all.
On page 83 of the DMG, Gygax himself suggested that DMs should assume the role of insane characters.
Well, that's no fun.
Insanity should have real, immediate, in-game mechanical costs that make dungeon-crawling more difficult. Having an insane character should be an irritating condition that you'll want to be cured of, much sooner rather than later. In-game insanity doesn't have to resemble real world insanity. Your character is going mad and simply isn't functioning the way they're supposed to. Which leads to...
Practical Insanity
1. Amnesia: You've forgotten just who exactly you are and now function at half of your current level as far as, to-hit, saves, spellcasting, and turning is concerned. Or -2 if you're 1st level.
2. Kleptomania: You now have the pick-pocket ability of a thief of half your level (even if you're already a thief, because now you're sloppy about it.) Any time the party takes a rest, you must save vs spells or attempt to steal a random item from a random party member. If in town, you must attempt a theft at least once per day against a randomly determined target. You will always deny your theft (but you might have one person that you secretly brag to about it.)
3. Catatonia: Melancholia to the extreme. When a stressful situation occurs, save vs spells or retreat into yourself. You won't do anything. You can be carried. Lasts 2d4 hours.
4. Schizoid: You think you belong to another class. Determine the class randomly (use the 4 archetypes, 1-fighter, 2-wizard, 3-thief, 4-cleric, reroll if you roll your class.) You fight and save as if you belong to that other class at half your current level. Wizards and clerics must save vs spells in order to cast a spell or turn undead, but only if someone else reminds them that they are capable of doing so.
5. Delusional: You think you're better than you actually are and wonder why you don't experience better results. In combat, you attack with a +4 bonus but only do half damage. Targets save vs your spells at -4, but your spells only do half damage or only affect half the normal number of targets and only last half the normal duration. You only receive half of your XP earned.
6. Suicidal: If a trap is discovered, you must save vs spells or purposefully trigger it. If a chasm presents itself, you must save or throw yourself over -- hopefully your friends will stop you. You'll be the first into combat every time.
7. Schizophrenia: Like Schizoid, but the other class changes daily.
8. Panic Disorder: Your anxiety is extreme and your fear is all consuming. When a stressful situation occurs, save vs spells or retreat as fast and far away as you can get. Your anxiety builds with every successful save you make against this disorder causing any subsequent saves to suffer a cumulative -1 penalty until at last you're overcome by fear and flee. The DM should make you call split second answers to right, left, or straight as you race through the dungeon away from the danger and the other PCs until you are alone and lost. Only then might you get a grip.
9. Melancholia: Save vs spells every morning to see if you have the will to live. If not, all rolls suffer a -5 penalty. Save vs spells to see if you'll even try to help in any given situation.
10. Obsessive/Compulsive: You insist on doing things a certain way even if it doesn't make sense. A fighter will switch weapons every round or with every foe or will insist on using his least effective weapon, how else will he improve? A wizard will cast spells in a certain order (who cares if you need fireball, it can't be cast before feather-fall!) A cleric will always waste the first round of combat in prayer or spend 1d4 turns ritually cleansing their equipment after every battle -- if they don't do this they won't be able to cast any more spells that day. A thief takes twice as many rounds/turns to do his thing, etc.
11. Manic-Depressive: On any given day, you're either (1-3) Hysterical or a (4-6) Melancholiac.
12. Hallucinatory: In combat there is a 25% chance, after you've resolved an action, that you did so against a foe that isn't really there.
13. Sado-Masochism: In any given combat, you're either a (1-3) Sadist or a (4-6) Masochist.
14. Homicidal Mania: You will murder any NPC that you think has failed you, lied to you, betrayed you, or even looked at you funny (yes I know, this is every PC that has ever existed. God Damn murder-hobos!) Even other PCs need to look out.
15. Hebephrenia: Any time a stressful situation occurs, you must save vs spells or behave as if you are under a confusion spell for the duration of the stress. Someone may attempt to snap you out of it with physical contact (slap!) in which case, you get another save.
16. Paranoia: You don't trust anything, every door, staircase, chest, ladder, every thing is a potential trap. Your movement is cut in half. Actions take twice as long. e.g., picking locks, searching rooms. Every NPC is lying to you, anyone who thinks otherwise is a complete idiot. You must save vs spells to be healed, because, fuck-no you don't trust their God! You will only eat rations and use equipment that you brought along yourself. And all gold is cursed...
17. Hysteria: Any time a trap goes off, or a party member dies, or you run out of rations or light, or you're lost, etc., you have to save vs spells or FREAK THE FUCK OUT!... causing an immediate random encounter check at +1 probability (most likely now a 2 in 6 chance.)
18. Sadist: When you slay someone in combat you will keep on attacking that target until it becomes pulp (this includes casting spells on it.) Save vs spells to snap out of it.
19. Masochist: When in combat, you must save vs spells in order to attack, otherwise you just stand there and take it.
20. Psychosomatic Disorder: You keep insisting there is something wrong with you. Don't you see these sores all over my arm? And soon there will be. You must save vs spells or terrible sores start to appear on your body. They ooze puss and they bleed and you must save everyday to avoid this terrible malady from worsening. Every time you fail a save, more sores appear and you lose 1 hit point from your hit point maximum until you die or your insanity is cured. Once cured, your original hit point maximum will be restored minus 1 hit point per week afflicted.
On top of the above afflictions, all insane characters should suffer nightmares. A save vs petrification is required each night to acquire any benefits of a full night's sleep.
Only heal, restoration, limited wish, and wish spells can heal these afflictions. Natural healing is possible but requires two successive saves vs spells done on a monthly basis, one save at the end of each month.
Or something like that.