Even amongst the most knowledgable of mortals, little is known of the whys and hows of the afterlife of Gemnos, but this much is known.
When a creature dies, its soul travels to the Underworld or it has sold its soul, in which case it goes directly to Hell. Sages speculate that this is why demons and devils work so hard to corrupt mortals. By striking a deal with a mortal, they bypass the 100 years of negotiations and possibly also steal souls they wouldn't have gotten otherwise. The only other exception to this is when a creature is killed by Destruction or Disintegrate, in which case the soul is also destroyed.
After the ten days of traveling through the veil, the soul arrives in the Underworld. Why it takes a soul 10 days to pierce the veil, no one knows, but those 10 days is the window during which a soul can be brought back by the use of Raise Dead. It is however impossible to contact a soul while it is passing through the veil.
Then, for the next 100 years, the soul will roam the Underworld as an incorporeal ghost, during which time Heaven and Hell will bargain with Cyric for it. If the soul has unfinished business, it will usually already be able to pierce the veil and return from time to time to the mortal world and haunt it.
At the end of the 100 years, the soul will either become as a creature of the Underworld (and possibly continue to haunt the mortal world) or travel to Heaven or Hell and be reborn there. It is during those 100 years that a creature can be brought back to life by the use of Resurrection and contacted using Speak with Dead. Once a creature is reborn, nothing short of divine intervention or True Resurrection will enable someone bring it back to life. Even with True Resurrection, there is little to do once the soul has grown used to its new body, something which happens another 100 years down the line.