Mind Rot
Mind Rot is a designed disease, created by some eldritch horror from the subterranean. It is said that Mind Rot was originally created as a means of getting human specimens for this monster’s dark experiments, without having to venture to the surface itself.
Mind Rot is actually a series of parasites that lodge themselves into the brains of their victims. It takes 1d3 days for the Mind Rot to take effect, and if the victim at any point receives a “Remove Disease” or similar magic, the disease vanishes. Once the Mind Rot is fully integrated into the brain, however, it becomes much more difficult to remove.
Every day the victim must make a saving throw, DC 20, or take 1d6 intelligence and charisma damage. None of this damage heals by non-magical means until the victim is healed of the disease. Until the first time the victim fails one of these saving throws, there are no physical symptoms. However, once the victim fails a saving throw the Mind Rot manifests itself as a hallucination. It can take any form, but it tends to take a form that is significant to the victim. If the victim was abused by his father, for example, the Mind Rot might take the form of the father. If the victim favors elvish women, the Mind Rot might take the form of an Elvin woman.
The Mind Rot will try to manipulate its host. It might try to convince him to attack one of his friends, steal, or otherwise do something out of character. For every evil action the host takes in a day, he takes a -1 penalty to the next saving throw against the disease. The penalty is increased to -2 for actions that specifically violate some code of ethics the host has.
In addition, the Mind Rot can try to taunt the host. The host must make a will save, DC 17 + the combined minuses for taking evil actions that day. If the Mind Rot’s insults are particularly piercing, the host can be assigned morale modifiers up to a -4. Friends of the host can try to help, and if their role-playing is particularly good and inspiring, the DM may award moral bonuses up to a +4. If the host fails this roll, it takes a -4 to the next roll against the ability damage. Note that the Mind Rot does not use this tactic right away, it tries to befriend and manipulate the host first and only if that fails does it take this more hostile approach.
If either the host’s wisdom or charisma reaches zero, the Mind Rot personality takes over. The player becomes an NPC with all the same class levels and abilities (save for those that would be taken away as a result of this change, like a paladin’s special abilities), its alignment changes to neutral evil and its intelligence and charisma becomes 16. The Mind Rot personality retains all memories the host had, and sometimes even masquerades as the original host. Different variations of this disease have different goals, some try only to spread, others try to return to their master, and some go out to gain new information and power.
While the Mind Rot is struggling for the host’s body, it can be removed with a Remove Disease or similar magic. However, it is considered to have spell resistance against this spell, DC 18 + the charisma or intelligence drained from the host, whichever is higher. In addition, it is allowed a saving throw against this kind of magic, at +5 + the charisma or intelligence drained from the host, whichever is higher. If the disease succeeds against the saving throw, it cannot be affected by this magic for another day. Once the Mind Rot takes complete control, however, nothing short of a Miracle or Wish can remove it, and even then the Mind Rot is allowed a saving throw. Even if the body is killed and then raised, the Mind Rot will be the mind that retains control, not the host. Mind Rot is spread through saliva, usually by biting another creature. The DC to resist is 20.