Hello everyone. I am about to run a campaign inspired by Michael Moorcock's ideas about Law and Chaos battling it out across the multiverse. My players will find that the world they are in is increasingly chaotic in small and large ways. I have homebrewed various Chaos creatures but what I am a bit stuck with is a way of tracking a cumulative effect of the exposure to Chaos on my player characters. I was thinking that some sort of wisdom-based system would help determine A) is there any effect on a player, B) what is the effect and C) does it mean they slip down a 'Chaos scale' and become slightly more under its influence each time they fail a check. The more I think about it, the more chaotic my mind feels.

The DMG has a section on "Madness" and grading its effects but you could also use the mechanics set out for the various Demon Lord Lairs, they all involve a Wisdom based save to resist going mad and even give you a random table with some very generic terms that players can then interpret as they see fit. To burrow from other systems you could also look at Call of Cthulhu which have a sanity mechanic that could be usurped into D&D as well.

a few things come to mind like, decreased impulse control. if they have a familiar or pet (which the DM should be controlling as usual) then you can add flavor with their actions and pass a note to the character asking them to follow suit. additionally you might roll dice behind your screen and upon a '1' you let them know an impulse has taken them to do a thing... chug a beer, yank someone's hair, lift a goat, refuse to go further until they can get their flipped dagger to stick in the ceiling, etc. something immediate. nothing like kissing another player or murdering innocents although you absolutely should not reign in their apprehension and discussion of it potentially escalating in that direction. impulse control can continue to be random or triggered as befits your campaign as it matures. this impulsiveness might be a long term effect that increases over time or it might be a localized indicator that something's wrong here and now.

also, adjusted luck. in a fight against something oozing chaotic aura you might surprise combatants by calling '1's and '2's as critical successes. "the arrow whizzes past the thing only to 'pa-TANG' off something metal and thunk painfully into it's back!" oh, and encourage some players to start using their gaming kits! maybe offer early on to give inspiration to anyone who participates in the group dice game during short rests (and then allow expanded use of inspiration for lucky things like turning a hit into a crit or canceling an enemy's crit). later then you can play out a dice game and manipulate one of the players (or all of them, why not) into rolling the same result over and over. "large straight again?? what are the chances??" "arg. it's like i can only roll '2's today or something."

Thanks for your reply. This has given me plenty to think about. I like the idea of random impairments to dice rolls, actions and maybe initiative. On the flip side I really like the suggestion of inspiration. 

I have read the madness section and concluded it wasn't quite right. Thanks for the Cthulhu idea - I will check it out. I think it may well work for me.

Update and thanks. I have created a system similar to the sanity pool in Cthulhu simply by rolling a d8 and adding wisdom modifiers. Once these are banked players will roll wisdom saves of varying difficulties each time they are in contact with Chaos creatures and Chaos effects. If they fail they lose a point permanently. If they lose all points then they are subject to Chaotic effects - maybe beneficial, maybe not - each time they are confronted by Chaos creatures and effects. The effects aren't in themselves game breaking but they may prove to be a hindrance or a boon. Could there be a way to regain points? Maybe. Thanks for the inspiration.