The Afterlife is a 'world' parallel to the living world of Medius and existed first. It is the location of the home realms of the three gods Mother Gaia (Heaven), Father Syndel (Hell) and Death (Purgatory) as well as where the souls of Medians are sent (or brought) when they die. By its nature, the Afterlife is ethereal so physical laws can behave oddly and behave significantly differently in different places. Sizes and distances are not necessarily fixed and there is no natural cosmology like a sun or moon. Beings in the Afterlife do not need to eat food to survive although this can vary from location to location. A notable oddity is that the souls of dead Medians can be killed but it only applies temporarily, they simply reappear back to normal the next "day"; this relief does not apply to the native Demons and Angels who can be killed permanently.
For Medians, the process of dying is not explained in detail but it appears that Death will collect the deceased soul and bring them to Judgement in Purgatory. He will then perform a short ritual to decide which location within the Afterlife that the soul will be staying. The process involves a seemingly magic book that lists the name of everyone and their registered destination when they died, like a reservations ledger. Rarely the book will declare "undetermined" as the destination which requires Death make the decision himself manually.
All souls that have been dead for more than a certain (unknown) amount of time, regardless of destination in the Afterlife, are supposed to reincarnate and be born again on Medius, creating a soul cycle. Death is supposedly responsible for recollecting the souls and performing a ritual for this as well.
The rough geography of the Afterlife is that Purgatory is in the "center" between Heaven and Hell separated from both of those by the River of Lost Souls (the Styx). The Styx appears to have a spatial warping effect, Death can cross the river in his boat but souls that have been delivered to Hell cannot swim across it back to Purgatory. Either there is a powerful current that the boat is immune to or the river is infinitely wide which would prevent flying across as well.
Death is seemingly the most influential being in the Afterlife as he is responsible for delivering souls to both Heaven and Hell as well as controlling Purgatory directly. He also created the guardian creatures Cerberus to guard the Gates of Hell and Drazil to guard the Gates of Heaven.