Death

Death refers to the permanent cessation of life functions in a living organism. It marks the end of the physical existence of an individual and typically leads to a separation from loved ones, memories, and the world as they once knew it.

Reincarnation

First Reincarnation

In the world of Toriel, death marks the transition of a mortal's soul from the Material Plane to one of the outer planes. The specific outer plane that a soul reaches is determined by the individual's alignment during their lifetime. Whether they lived a good and virtuous life, or one filled with evil and malice, their soul will find its final resting place in an outer plane that reflects their actions in life.
Individual who sold their soul, such as using a devil's contract, can have their soul transported to an outer plane not corresponding to their alignment as an effect of said conract.

Second Reincarnation

In the world of Toriel, when a mortal dies, their soul travels to an outer plane based on their alignment during life. However, once reincarnated, if the individual were to die again, their soul would become lost in the void with no further chance of reincarnation.
There are two exceptions to this rule. Firstly, if the reincarnation was under the control of a master, such as through a devil's contract or an angel allegiance to a deity, the soul may be reincarnated again. Secondly, souls that are lost in the void may end up in a plane known as Echo, where they take on their last physical form and slowly turn to stone, becoming trapped as statues for all eternity.

Resurection

Spells and magic effects aimed at bringing a mortal back from the dead only work as long as the soul has not yet reached the outer planes or turned into an undead. Once a soul has reached one of these states, resurrection becomes impossible. It typically takes 10 days for a soul to reach the outer planes after death. However, in some cases, the process may be hastened by powerful magical forces. In these instances, the soul may reach the outer planes within mere hours of death. This makes quick action a necessity for those wishing to bring a loved one back from the dead. It is important to note that resurrection spells should only be attempted by skilled and experienced casters, as the consequences of a failed attempt can be dire.

Modified spells

Spells such as Resurrection, or True Resurrection will only work within 10 days of a creature's death.

Undead

Ghost, on the other hand, are created when a soul refuses to leave the prime material realm, either through the use of powerful necromantic magic or by willpower. This creates a state of undeath, where the soul remains bound to the material realm, no longer bound by the laws of life and death.
Similarly, undead are created using powerful magic, and are either soulless bodies, or cursed souls. These undead creatures can range from the reanimated corpses of the recently deceased to powerful liches who have extended their undeath over centuries. While they can often be found roaming the prime material realm, their souls are never reincarnated and are lost to the void upon their destruction.