Ghost
A ghost is a half-ethereal half-material being, a soul of a dead man returning to the material plane. The reasons are usually very violent, humiliating or for some other reason suffered deaths, which the person denies to accept. Instead of joining the afterlife, he stays in a limbo until some force allows him to travel back to the material plane to finish its business. The problem is that usually the same force that brings them back also controls them to do their bidding. The ghosts will always try to fight its control, indirectly acting against the controller, but are unable to act directly against him. Should a ghost be killed, its soul is effectively destroyed.
The ghosts can walk through material structures, but not through material live beings, for their own souls act as a barrier. Water slows them down, draining their energy (but not damaging them).
They could be said to be rather weak, but not many types of damage can hurt them. They have very high Damage Reduction against most material-based attacks, and are only vulnerable to fire and magical attacks.
A ghost may wield a phantasmal weapon, if he had one in life, dealing normal physical damage with it, or can use a draining touch, with which he can heal himself.
Never can ghosts leave a certain area they are bound to, unless someone controlling them orders them to, and even in such case, it will cost them, as if they were moving through more thick air.
A single ghost should not be extremely dangerous, though it would be very hard to kill by physical means. A pack of ghosts, however, can severely threaten lives. Explaining: an adventurer finding a ghost in an old manor might be unable to kill it with only a sword, or a small torch. It is unlikely that the ghost will be able to pose a real threat for his life, unless he acts with extreme carelessly. A regular priest or wizard should be able to deal with such creature. An adventurer in the middle of a cemetery at midnight discovering that is the time the whole buried dead men awake as ghosts, on the other hand, will find himself in an almost-sure-death situation.
Walking Corpse
A walking corpse is a an animated corpse of very limited intelligence, lower than most animals. It's sole purpose is to seek living flesh, but it's a doomed quest, for as soon as he strips the life out of their targets, they see a new undead before them, and they have to keep seeking a new food source. Its intelligence is similar to that of a mammal animal.
They can survive in rural areas by hunting animals down and feeding on them, while also attacking unwary travelers. In more densely populated areas it is unlikely to find them, unless contained by necromantic magical means, or in large hordes ravaging the land.
Walking Corpses are particularly resistant to piercing and bludgeoning attacks, being more easily hurt by slashing them, burning them or by magical means.
Walking Skeleton
A Walking Skeleton is a being with no purpose or motivation other than it's masters'. He won't act or move unless having received an order to in the given context, but never on its own initiative. It can be seen as the simpler undead, and it works with absolute loyalty to turn into reality the requests of the Necromancers that usually raise and control them. They need to receive rather simple commands, like "kill everyone who passes through the door" or "kill all the goblins", but cannot understand more complex demands like "kill all those who threat the safety of the princess". On the other hand, for its deeper connection with the controller, it can get implicit orders. In that sense, if the controller knows how to pilot a vessel, it can order the skeleton to "sail", and the skeleton will understand what the controller understand for "sailing". If the controller has no idea of how to navigate, the skeleton won't be able to do more than what the controller might attempt by himself. Its own individual intelligence is similar to that of an insect.
They are rarely found without a master, in which times they can be easily confused with an inanimate pile of bones, and normally are found either in the lair of Necromancers, be it in the reclusive wilderness or in dark corners of cities.
Walking Skeletons are particularly resistant against piercing and slashing attacks, but are extremely vulnerable and can hardly sustain much bludgeoning damage. Magical and burning attacks will also hurt them quite badly.
Lich
A Lich is a being of immense power, and can be seen as semi-gods. It is the voluntary abdication of life for an eternity of negative-energy-fueled unlife, and seemingly unlimited power. They can drain life from people like a ghost, have an immense magical repository, and usually those who reach such state they also have very vast magical knowledge and resources.
An extremely rare sight, a Lich's presence is never unnoticed for long. They can even destroy realms with their vast power. For they tend to be chased and destroyed by their neighbors, since they are a constant threat, they are usually governing territories with low to no alive population, in hard-to-access lands. That is, of course, when they are not governing large empires of subdued mortal populations.
A Lich's body will be similar to a very well preserved Walking Body, and if sustaining enough damage will decay into that of a Walking Skeleton. Draining enough life, he'll be able to regenerate it back into the better state.
