There exists the Universe with its stars and planets, and then there exists a barren copy of it that occupies the same place at the same time: the Underverse. This is where souls (or minds) go once their bodies die.
Travel between the Universe and the UnderverseEdit
Ordinarily, the dead find it impossible to cross the barrier between planes and return to the plane of the living; even if they know how to do so, the Underverse lacks the arcane energy necessary for such a task. The living, however, occasionally become powerful enough to be able to surmount this barrier; their purpose when doing so is often to bring a loved one back to life or to gather undead servants.
The condition of death leaves a permanent and ever-strengthening mark on those who pass on. When a creature dies, this mark, usually called Taint by scholars, at first invades it, fighting for control. It spreads within the creature's soul like a disease and consumes it slowly; a creature brought back from death is never the complete creature, but rather a diminished version of it, unless the Taint is somehow removed in the revivification process. Eventually, the Taint invariably wins the battle, and the creature's exhausted soul embraces it. The soul feeds from it and grows more and more attached to the Underverse, making it increasingly difficult for a living creature to return it to the other side. This also makes the soul more powerful, since more time dead means more time feeding from the Taint.
The region of the Underverse corresponding to the Earth is called the Undearth. The dead sentients who roam this place have developed a language of their own which few living creatures have ever heard; those few who have heard it claim it sounds like death itself.
The greatest necromancer in history is Zahar, considered a god by the sentient mortals of the Earth. He alone has ventured into the Undearth and made his way back on his own (or with a little help from Mordun, according to some accounts), and he alone is able to remove the Taint from a dead creature as he brings it back to life, thereby preventing the condition of undeath.
Throughout the Third Age, other people have learned --either directly from Zahar or by studying his book, Open grave-- how to tap into the Undearth and draw from it. Vekumm's own immortality stems from permanently inhabiting the barrier between planes, never stepping fully into one or the other, a trick he developed after working closely with Zahar for several months.
Vekumm and Zahar's countless experiments with death and undeath, as well as their endless fetching of undead to serve their divine purposes, have weakened the barrier between the Earth and the Undearth. When they were trapped by ambitious mortals, there was nobody left to keep an eye on the dead and prevent them from crossing over to the plane of the living, so countless undead began to roam the Earth. Although they include creatures of all kinds, the most noticeable are sentients, who despise untainted life and seek to consume or destroy it entirely. Because of this and other things, the Third Age is an age of darkness and peril for the mortals of the Earth.