After death, a mortal's spirit travels through the ground to the Underworld, which lies a long way beneath the surface of the planet. The spirit is escorted by the horseman of Death or another one of his servants, or very occasionally Death himself. There, they are judged based on their past deeds and placed in a section of the underworld accordingly. Of course, these sections correspond to the division of the tetrahedron by the leylines, so someone who worshipped War and caused much pain during their lifetime would be placed near the titan of War.
The Underworld has seven layers in total. Depending on how much an individual was aligned towards a certain deity, they will be placed at a certain depth. The deeper they are placed, the more aligned towards that god they were. This follows because the gods themselves take residence in the center of the planet, so the places closest to them are taken by the most devout followers, though there is a LARGE distance between the seventh level and the residence of the gods.
Understandably, the Underworld is very big. The spirits of the dead spend their time partaking in activities relevant to the area they have been assigned e.g. spirits under the God of the Unknown might spend their time playing Cluedo or some such. Travel between the areas is possible, but since spirits can only fly at about twice their worldly running speed, it takes quite a while. Within 24 hours of dying, a spirit may also challenge the god of Death to a contest for their life, but there is a long waiting list for this process (the 24 hour restriction only applies to getting in line for it). The god of Death allows this because he enjoys games and such. Because of this, there is no particular penalty for failing other than not being able to try again. The actual game to be played is decided by where their spirit was placed.