Each living creature has a soul and this soul occupies the body. When a creature dies, the soul leaves the body.

However, the soul is still connected to the body it belongs to by one or more Soul Strands. Because of this connection, the returning of the soul to its body by resurrection magic is possible. The stronger the soul, the more Soul Strands it has. Most sentient races, like humans, elves, halflings, orcs, brokanian and so on, have ten.

Note: in NWN this can be increased to fifteen by taking a feat called Soul Mother Defense.

At the moment of death, the soul runs the risk of a Soul Strand snapping. When the last Strand snaps, the soul is no longer connected to the body to which it belongs. The creature can then no longer be brought back to life by magical means and is permanently dead.

The Harvester of Souls 

When a creature dies, the soul leaves the body and starts to travel towards the Thread. It is still connected to its body by one or more Soul Strands. The soul will first end up in the Plane of the Lost, the place where the Sheet and the Binding overlap. Here, the soul will be judged by a creature called the Harvester of Souls, also known as Harvester for short. This entity resides in the Plane of the Lost. It is not a deity. Some think the Harvester is really a god of death but others do not think he should be classified as such since he just passes souls on to their next destination.

It takes three days for the judgment by the Harvester to be made. If resurrection magic is used on the corpse within three days of a death and the soul is still attached to its body by one or more Soul Strands, the soul is pulled back and returned to the body.

If no resurrection or bindstone magic is used, what will happen then depends on the faith of the soul:

1) If the soul followed a deity, it is cast into the Heavens (which border the Binding Void) by the Harvester after the three day period. It will then move to the section of the Heavens that is the domain of the deity it followed and spend the rest of eternity there.

2) If the soul did not follow a deity, two things can happen when the three day period is up and judgment takes place:

     a. In case a deity wants to claim the soul as one of its own, for example because the actions of the soul in life reflect what the deity stands for even though the soul did not follow the deity, the deity can mark it and the Harvester will cast the soul into the Heavens when he judges the soul when the three days are up, just like souls that did follow a deity.

     b. If no deity claims the soul, it is cast into the Binding (which also borders the Binding Void) by the Harvester when the three day period is up. There the soul will then migrate through the elemental planes and the Pits of Endless Strife towards the Thread, which will absorb the soul.

During the three day waiting period, a bargain can be struck by somebody visiting the Harvester, allowing for the reattachment of a single Soul Strand and making it possible for a soul to return to its body. The way to the Plane of the Lost and the Harvester is perilous, though, not to mention the bargain which differs every time. Those who perish in the Harvester's domain instantly have all their strands cut and are permanently dead

Souls and the Pits of Endless Strife

When souls that did not follow a deity or were not marked by one as they waited for judgment are cast out of the Plane of the Lost into Binding Void, they migrate towards the Thread.

However, to reach the Thread they need to go through the Pits of Endless Strife. The nefarious creatures of the Pits will always want to capture souls to gain more power. The more souls one of these Pit Lords can capture, the larger its domain will be and the more powerful it will become.

Furthermore, the master of a Pit always has no shortage of competition from others in his realm, and even if he misses the soul another Pit denizen will gladly consume it and gain a little more power. Consequentially, any soul passing through the Pits of Endless Strife is highly likely to be devoured.

Bloodpools and Blood Wells 

It is thought that Bloodpools and Blood Wells somehow touch the Plane of the Lost and therefore when you die near one you will lose a Soul Strand.

Death on the Plane of the Lost

If a creature dies on the Plane of the Lost they are instantly lost forever (i.e. permanently die) and are judged by their deity or the Soul Mother instantly.

Reattaching Soul Strands

It is said that if a character is ever permanently dead that brave adventurers could travel to the Plane of the Lost and deal with the Harvester in order to get their soul re-attached to their bodies (i.e. one Soul Strand). This would surely be a deadly and highly dangerous quest however and it is rarely attempted. Even souls that have traveled to their deities they must return back through the Plane of the Lost so one must still deal with the Harvester.

Note: In NWN, Soul Strands can also be returned via the Graceful Plea system.