Upon death, every mortal's soul makes the journey to Between. After a time drifting in that plane, the soul would then make its way to the plane of one of the four elemental gods, determined mainly by the mortal's beliefs and faith when they were alive.
Occasionally, more than just the mortal's soul would travel to Between, and sometimes the extra essence would form into a semi-physical being. These beings were known as undead. They would remain on Between for hundreds of years if not for all eternity. It was not exactly known why this happened. However, there were several theories. Particularly powerful mortals had the will power to transcend death itself; hence, a more significant portion of their essence would remain intact. Then some mortals took part in rituals and dark magic that helped them cheat death to produce the same effect. In both cases, they would become undead.