Does it Feel Like the Walls are Closing in to You?

Over on the Book of Faces, someone asked about adding a sort of madness mechanic to an OSR game like B/X D&D that would give you the sorts of character attributes that you see in Darkest Dungeons, like becoming selfish, irrational, or hopeless.  And of course I thunked some thoughts on this.

I'd make it a resource like hit points.  Give bonuses to it based on Wisdom and maybe also Constitution.  Like hit points, you roll for them and what you roll is based on your class.  Spell-slingers like magic-users, clerics, and elves roll the lowest dice, and expert/rogues and halflings roll the highest dice.

I'd rename it Reason.  Taking damage, witnessing horific events (like things happening to your companions), etc. lowers it, just like attacks and traps lower your hit points.  If Reason reaches zero, you roll on a table of afflictions like those in Darkest Dungeons, but you'd absolutely use this part to flavor to your group's taste.  I'd make the list have the least affecting afflictions on the low end and the most impactful at the high end.  Then, every time you roll again on the table, you add another +1, or maybe you roll more dice, starting with a single d4 or d6, but adding another d4 every time you roll.  Once you'd acquired an affliction in a session (or possibly location), you couldn't acquire another one until next session (or after you'd left the location for more than a day).  

I'd give spell-slingers a bonus on the power of their spells based on how low their Reason is.  The lower the better for those who seek to warp reality with eldritch sorceries!

You could have a lot of fun with how Reason is replenished.  Rest, sure, but the quality of that rest should be taken into account.  Sleeping in the dungeon in your armour?  Maybe a point if that much.  Sleeping in a ditch?  Maybe 2 points.  But if you've got a nice cozy room at an inn and had a good hot meal, you get lots of points back.  Halflings might replenish just from eating.  

Maybe you can replenish reason by telling in-game jokes, listening to music, attending religious ceremonies dedicated to gods of goodness, or carousing.  Sex and intimacy might be healing along these lines.  Having a deep emotional bond with someone might have a protective effect, but if something happens to your special someone, it might hit you just as hard as it hits them.  There are all sorts of neat directions you can take this.