Death and the Underworld

"Fear not Omal, for your deeds and skills will aid you in times to come," ancient Nyk said unto the hero. "Your way to Yeno is prepared, but the journey is dangerous."
Omal spoke, "The underworld takes as many as it can, Chaos' final assault against the creations of the Great Three."
Nyk nodded, "Just so. Chaos unrelenting, tries to take all mortal life from Set and I. But your friends and family have followed the ancient rites. Your passage to the underworld is secure. I shall guide you for a time. But the final trial is yours to face alone."
"I fear nothing Goddesses of the Grave," Omal said.
"It is well to say that. But your must prove it with actions. The Underworld is unlike anything upon beloved Cadash. Come with me, and see the true nature of mortality's end." With that the goddess held out her ashen grey hand, her milk white eyes seemed to glow and she took Omal with her to the Underworld to face his first trial beyond death.

Summary

The end of the mortal life, journey to through the underworld and uniting with the Great Gods is the oldest story upon Cadash. Nearly all cultures and races have some variation of the story. Elves and the children of Ashtyr have legends that are very similar indeed. While the children of Kor have a varient that shifts some of the elements around.
When a mortal dies, funeral rites must be given within a certain number of days (Kor's children have three, the other races have five). If the Guide of the Dead (Nyk in most cases) is not appeased, then the soul will remain tangled in the flesh, and will have the potential to rise again as the undead. If the rites are appeased then the Guide will take the soul in hand and bring them "across a great expanse of water and unto a the great island gate". There at the threshold of life and death is a guardian, ancient and foul, who keep mortals from passing. The Guide and her charge may pass and decend into the underworld.
This world beneath Cadash is vast and seemingly endless. However, it is said that those souls who follow the guide's advice and pass the trials will eventually move beyond and until the final realm, the home of the Gods. There they will be united with the worthy, souls of heroes from the past, as well as the loved ones who did not fail the trials. For some this realm of the gods is a paradise of eternal blisss. For others it is a time to prepare for the final battle against Chaos when Cadash finally succumbs to the end.
The Underworld itself is varied, strange and ever-changing. Nearly every tale adds a new facet to the landscape, and removes a few that were once popular. There are a few features that remain in each iteration.
The Gate of Ancient Bone is the entrance to the Underworld. It is an enormous gateway made from the bones of the first beings upon Cadash (some say dragons and giants). The gateway is open to those who would enter, and have passed the ancient guardian before it. As long as the Guide is with the soul it can pass. But mortal seeking to enter will find a puzzle before them. Few of the living have ever pass through the Gates of Ancient Bone.
The Forest of the Lost is where souls who fear to continue past the trials and those who have failed but not been destroyed end up. It is a vast fungal forest, bioluminescent and hazy. It is said that souls can wander endlessly here, eventually fading into the very haze of spores and mist and never seen again. This is a place of tranquil oblivion in some tales and in others a devious trap filled with a noxious minion of Chaos who devours the souls when they least suspect it.
The River of Endless Night crosses the underworld. Here a soul must pay to cross the river. Tales vary, but there is always a price. Sometimes there is a bridge to cross, or a ferry to board, some say you can walk through the waters, but they will destroy you if you do not pay. The waters are said to look like a night sky in motion, beautiful and disturbing in equal measure.
The Spires of Living Stone are said to be the home of beings made from rock, soil and mud. Here is a battleground between the forces of Nature and Chaos. Crossing through these spires is always perilous and many souls are utterly destroyed by either side of the conflict. Many stories claim that passing through the Spires is one of the trials of the dead.
The Labrynth of Onyx is also called the Palace of Death. It is here where the god Set rules over the Underworld with his queen Nyk. The palace looms over a massive maze of gems, rock, fungus and ice. Those who wish to speak with the King of the Underworld must pass through the maze. It is said that all souls wishing to see the Realm of the Gods, must obtain the blessing of the royal pair. The maze is infamous for its monstrous guardians, devious traps and bizarre illusions. Only the most worthy in the eyes of Set can continue on.
The Sea of Chaos is where the most chaotic of souls end up. They are drawn to the endless shifting energy and power in this place. It is said that those souls who embrace the chaos become fused with greater powers and become the monsters that terrorize souls with the Labyrinth or in the Spires. Others are sent back to Cadash to work new acts of destruction. A few heroes have attempted to voyage upon the Sea of Chaos and face the dangers there (and gain Set's approval). It is also said that there is a way to Cadash somewhere upon this sea.
The Mountain of Ascension is the final stage of the journey through the underworld. Here the souls that pass their trials climb up and into the the realm of the Gods. It is said that the mountain shimmers like metal or gems and seems endless in it's height. Those with faith in their destinies will find a summit, but those who doubt are wandering there to this day.
The Land of Shadow is a strange twilight realm between the dead and the living. It is said that this is the home of the Shadow Elves. It has entrances into all areas of the Underworld and it is said that Shadow Elves themselves may be lost elven souls who can not rise onto Cadash without protection from Set.