Restless spirits are those killed dishonourably or for no reason, in wars or famines.
In their Hadean form, the lesser Dead are ghost-like beings resembling their living form in features and size. However, if drawn into a corpse they will possess the appearance of that corpse, being in control of it.
They are sapient, but are bound to their summoner's will when returned. The sensory deprivation and anger of being dead often makes them mentally weaker and mad.
As they are not truly alive, it is very hard to incapacitate them. It can be done by decapitation, magical unbinding, setting fire to them, or killing their summoner. They have no need to eat, drink or such things, but feel a rush when they cause a spirit to leave its body, due to the wake it produces in the Hadean plane.
They are still organic entities, and have no vital signs, so are slowed much by cold and rot away much faster in the heat. Once their bodies are in pieces, their spirit is forced back into Hades.
They cannot cross running water, and are mostly afraid of sunlight.
The Greater Dead are the restless spirits of those who were extremely powerful in life, and were strong enough to keep themselves within Hades rather than passing on. They are extremely powerful, able magicians possessing free will and a usually malevolent intent, but are slightly less mighty than they were in life due to the power they must exert holding themselves in the real world.
When possessing a body, an dark aura will surround them and they will usually be able to produce certain trademark possessions they had in life from wherever it was before- probably by forming a connection with it.
All living species have their own ring of Hades, the more powerful being closer to the centre and Death. The circles of animals are further from the edge (and so far easier to access) than those of humans, and the circle with the Dragons is further still into the realm.
Spirits from more powerful beings can be put in lesser corpses, but not vice versa.
The Dead wish to destroy all living beings, or to become alive again themselves.
If the undead are destroyed, the restless spirit will almost always be deposited in the same location in Hades from which it was taken. However, sufficiently powerful banishment, or being banished within Hades, will lead to them being cast into the portal of true death.