I was looking to create a mental disease to infect my players after seeing some horrific events.
Instead of using the usual Endurance skill check, in it's place I was planning on using 1/2 level + CHA or Wis modifer.
Disease Progression: Once you’re infected, make an 1/2 level + CHA or Wis modifer after each extended rest to see if you improve, worsen, or maintain your current condition. A disease specifies two target DCs: a lower DC to maintain and a higher DC to improve.
Maintain: If the check result beats the lower DC but doesn’t beat the higher one, your condition remains the same.
Improve: If the check result beats the higher DC, your condition improves—move one step to the left on the disease track.
Worsen: If the check result doesn’t beat either DC, your condition worsens—move one step to the right on the disease track.
Cure: When you reach the left edge of the track, you are cured and stop making checks.
Final State: When you reach the right edge of the track, the final state of the disease takes effect. Once the disease is in its final state, you no longer make checks to improve. Often, the only way to recover from the final state is through the Remove Affliction ritual.
Heal Skill: An ally can use a Heal check in place of your 1/2 level + CHA or Wis modifer check to help you recover from a disease.
hm... this is a good question. In real life a mental disease is so hard to put a handle on, but if we look at the insight skill, in general terms it is to determine if something or someone is false. You could say that the mental disease is creating a falseself, as the mind begins to break and creates a false new personal reality.
Here is the mental disease I am working on.
Delusional Depravity Level 12 Mental Disease
You are convinced that the world is an illusion, you pay more attention to the whispers in your mind than to your surroundings.
Attack: +16 vs. Will
Insight improve DC 28, maintain DC 25, worsen DC 24 or lower
Stage 0: The target is cured.
Stage 1: The target begins each day with no action points. Each time the target becomes bloodied, it begins to listen to the voices in it's head and is dazed (save ends). Both of these effects apply until the target is cured.
Stage 2:The target can not gain action points or use action points. Each time the target becomes bloodied, it begins to listen to the voices in it's head takes ongoing 10 psychic damge and is dazed (save ends both). Both of these effects apply until the target is cured.
Stage 3: The target is catatonic and unable to take any actions.
Delusional Depravity Level 12 Mental Disease
You are convinced that the world is an illusion, you pay more attention to the whispers in your mind than to your surroundings.
Attack: +16 vs. Will
Insight improve DC 28, maintain DC 25, worsen DC 24 or lower
This sounds like a disease that most people will die from once they catch it, but that might be deliberate as a plot device. I think Heal should still alow for a mechanic to cure them but maybe they need to travel to learn how to cure this specific disease.
I would also be careful to make sure you don't have a player that has several feats tied into using action points as that would change their play considerably and may make them not like the game. Losing healing surges might be a good option as that seems more like a morale thing.
I think that if this was applied to my group their levelling would slow down considerably because they wouldn't want to get into dangerous combats. Also the party barbarian with his need to be bloodied for some of his powers to activate would be seeriously nerfed.
That said, it is an interesting idea and if your game isn't too heavy in combat I think it could drive some good roleplaying and questing. As an alternative you could take the mechanics away some how and not tell them that things are happening. Role play ghosts etc
The only caution I'd have is against using Daze as an effect. Daze severely limits PC action (and player involvement), almost always meaning less fun for the player during game. And when you reduce action points as stage two, you're pretty much condemning a player to sit out most of the fun until healed.
Maybe the voices in the head could be more of a dominating effect.....at early stages it's distracting (-2 to attack), later threatening (grant combat advantage all the time), later still there's a chance each round of taking AOOs on allies who leave your threatened area.....
Just my own two cents based on what my group likes. Make things happen, don't restrict things from happening.