Ghost Features (3.5e Variant Rule)
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Ghosts usually have powers related to the way they died - a ghost that was burned on the stake may be able to control fire and heat, a ghost that froze to death may be able to control ice and cold, a ghost that drowned may be able to control water, and so on. Plus, if a ghost already had some supernatural powers in life, these powers most likely grew stronger after their death.
As a rule of thumb, vengeful ghosts (ghosts who refuse to move on due to desiring revenge, usually on those that killed them or those they see as responsible to their death) are more powerful than standard ghosts, and violent ghosts (ghosts who refuse to move on due to desire to harm the living, which is often carried on from their lives) are even more powerful than vengeful ghosts. This is because a ghost's power is direct linked to its emotional state: emotions like anger and vengefulness drastically boost a ghost's power, whereas sheer malice boosts it exponentially. For instance, the ghost of an executed serial killer will normally be much more powerful and dangerous than the ghost of a drunkard who fell off a bridge.
Many ghosts who abstain from moving on for too long may become vengeful, as the passing years twist their memories and taint their perception with anger and frustration. Eventually, they begin to lose their sense of self and behave like wounded animals, attacking everyone that gets too close.
Banishing or Destroying Ghosts
Obviously, it is difficult to kill what is already dead. However, there are a few ways to take care of problematic ghosts. However, there are many kinds and types of ghosts, some possessing exclusive weaknesses that aren't possessed by other kinds.
Traditional Spirits is a collective namesake referring to certain kinds of ghosts that follow common rules. Traditional Spirits cannot touch iron (including cold iron and other metals that behave like it) and salt. A direct contact with either abolishs their manifestation for a short while (between 1 minute to 1 round, depends how powerful the ghost is), or forces them to leave a possessed body. They also cannot enter consecrated areas or they will be destroyed (powerful Traditional Spirits, and Traditional Spirits of Good/Neutral clerics, are immune to that effect).
Another group of ghost kinds are Relics, which are quite similar to Traditional Spirits, even possessing simialr weakesses, however are also extremely sensitive to silver (including alchemical silver, mithral and other metals that bahave similarly), which is incredibly marmful for them, and can even be used to ward them off. In particular, Relics retain little sense of self. Weaker Relics are mostly unaware of their surroundings and often trapped in a single, repetitive pattern of behavior. Stronger relics are aware and reactive, but for the most part still continue to repeat the same pattern of behavior and are unable to properly communicate with the living.
Most ghosts, including Traditional Spirits and Relics, can be forced to move on if their physical remains are destroyed. The damage inflicted unto the remains must be such that they are indeed destroyed; for instance, simply mutilating or dismembering the body isn't enough. Burning the body may suffice, if accelerants are used to maximize the damage. However, some ghosts also attach themselves to an object that was important to them in life, or even to an artificial structure (usually the one in which they lived or died). In such cases, this object/structure must be destroyed as well (burned, melted, shattered, etc.) in order to force the ghost to move on.
The only kind of ghosts that cannot be ward off or disposed of are Living Ghosts, because they are as close to life as they are to death. Burning their remains is ineffective as well, as their very existance attaches them to the Prime Material Plane.
However, all ghosts have one common weakness - other ghosts. Ghosts can destroy weaker ghosts by absorbing their energy, which is both forcing them to move on and leaving them such damaged they can never be resurrected via mortal magic; even characters with divine ranks have only 5% per divine rank they have to succeed. This damage is generally permanent, unless healed via direct divine intervention. If two ghosts fight, the weaker ghost will be affected similarly to what explained above. If the two are equal in power this will happen to both of them.
The only way to actually destroy ghosts for good is to use a method that destroys the soul (as ghosts are, in fact, souls with baggage).
Ghosts of Outsiders
While most Outsiders (except Native ones) cannot become ghosts after death, sometimes they are cursed of otherwise subjected to transmutation that converts them into ghosts while they are still alive. However, only very powerful (and mostly dark) spellcasters can do so; a Necromancer, for instance, may consider it a good method to acquire powerful (if resentful) servants.
Outsider ghosts are immune to burning their remains (since there are no remains to burn - their body was transformed) and might have a certain resistance, or even complete immunity, to other ghostly weaknesses, depends how powerful they were before they were cursed. Regardless, since most outsiders are powerful compared to other creatures, outsiders that were cursed into becoming ghosts are usually more powerful than common ghosts, and may have some unique powers and traits.