Once deceased, all souls enter the "Ethereal" (the "unconscious" directly adjacent to the Prime Plane) where they are presented with several options:
Option #1 - If a person was a follower of The Sovereign Host during life, Dara'Khul sends an Avatar of Death to escort them to the Isle of Driet, where, based on their life, is place in one of 4 realms: Bliss, Rest, Apathy, or Despair, each of which ruled over by an entity sharing the realms name.
Option #2 - If a person was a follower of The Silver Flame, their soul is absorbed by the flame itself. It is unknown what happens to the souls once a part of the flame.
Option #3 - If a person is a follower of The Dark Six, a collector is sent to drag their soul back to a realm of the Underdark where their soul is destroyed to give birth to a new evil creature.
Option #4 - In rare cases, a soul may remain on the Material Plane, by magical means, by immense determination of revenge or hatred, or by sheer force of will. In this case, the soul is vulnerable to total annihilation or becoming consumed. In these cases, the soul is obliterated and ceases to exist in any form.