The Underworld

The Underworld is the place of final rewards and endless suffering. Although many believe the Underworld is literally beneath the ground, in truth the plane is distinct from the mortal world, reachable only via magical means, hidden pathways, or by death. The souls of all dead sapient creatures awake on the banks of the Tartyx, where they meet Athreos the River Guide. From there, those capable of paying for his service are ferried to their new home in the Underworld.
Most imagine the Underworld’s wards as being stacked atop one another, but their actual relationships defy mortal understanding. While the Tartyx River reaches each ward, Phylias is typically defined as the entry to the Underworld, while Tizerus is farthest from this entrance. The other wards hold their own equal places in between. Regardless, souls destined for each realm reach their destinations with equal efficiency and permanency.
Each distinctive Underworld ward is effectively infinite in scale, with the space between noteworthy locations endlessly expanding and contracting. Traveling between locations is typically impossible on foot. Secret paths that defy mortal logic, magical steeds, and the intervention of powerful Underworld denizens all might speed one’s travels, though. Attempting to escape the Underworld is an entirely different, nearly impossible matter, though. 
Although vast beyond understanding, the Underworld’s most noteworthy regions are detailed here.

The Tartyx River

The Tartyx forms the boundary between the mortal realm and the Underworld. It is also known as the Rivers That Ring the World, as it is formed from the confluence of five tributaries, each originating in one of the Underworld’s five wards. The Tartyx is vast, with one far shore impossible to see from the other. Countless drifting islands dot the river, some forested by leafless trees, others heaped with crumbling ruins. Still others are the domains of powerful demons and strange entities that death proves not quite able to claim. None of these tiny lands are hospitable to either the living or the dead. Even the waters of the Tartyx hold their own threats, both mysterious creatures that slither beneath its rippling waters, and their own infamous power to wash away memories and all sense of identity.

Agonas

Agonas is the last home of warriors and soldiers who battled in life without honour, as well as the souls of cowards, mercenaries, and others who lived by the sword. Their cries of pain and triumph echo on the faintest wind, stirring bloodlust in all who hear them.
The clash of weapons echo over every ridge and canyon of Agonas’s rocky expanse. Great stadiums and platforms rise from the arid landscape, some carved from the remains of titanic ruins, others hanging from massive, rusting chains. The many theatres of battle blend with dwellings carved into natural rock towers, forming an extensive network of arenas and errant fortifications. At the ward’s heart thrums the Stadium of Dishonour. Here fierce soldiers test their blades against one another and against the teeth and claws of vicious monsters.
Upon arriving, dishonourable souls relegated to Agonas are met by oreads, the souls of other dead champions, and cyclopes, who guide them to the scene of their endless battle.

Phylias

The souls that occupy Phylias were uninspired and didn’t strive for greatness in life. They plodded through existence and left no mark upon the world through their deeds or their deaths. In the afterlife they perform repetitive tasks in close quarters with others just like themselves, all in a mockery of a living community.
Artless architecture of plain, grey stone gives Phylias an outward appearance of normalcy at first glance. But the buildings are titanic, blocky masses of hodgepodge construction erected seemingly without plan. Although the structures might be impressive in size or number, they are cold and hollow, derelict monuments in an endless slum.
Those who arrive in Phylias are met by no one. They are simply left to wander away and slip into a place among the shuffling masses.

Ilysia

In Ilysia the souls of heroic mortals and of those who died unjustly find eternal rest and comfort. Erebos’s grim influence holds no sway here.
Ilysia is a sanctuary of peace and tranquillity. Majestic temple-palaces stand amid lush forests, the colourful vegetation draping luminous marble in living mantles of flowers and sweet fruit. At the ward’s heart towers the Citadel of Destiny, where heroic souls gather to honour the gods, trade tales of glory, and feast with worthy companions. They also hold elaborate physical contests in which any Ilysian soul is welcome to participate.
When the worthy dead arrive in Ilysia, they are welcomed by dryads, majestic chimeras, or the souls of legendary heroes.

Nerono

Nerono is the final home of souls haunted by their memories and of mariners who were lost at sea.
Vast oceans fill Nerono, a realm dotted by lonely islands and crisscrossed by aimless ships. Titanic ruins and great, algae-slick chains rise out of the sea, as do the weathered hulls of legendary shipwrecks. The sky is a misty blur of colour that hangs over water as still as glass. Despite the ocean’s normally placid appearance, mighty storms often arise from nowhere, casting souls into waves and whirlpools by the scores. Somewhere within the great oceans hides a twisting tangle of tides and winds called the Labyrinth of Memories. This maze of waterways confounds unwary travelers and twists their course into an inescapable path for eternity.
When a soul reaches Nerono, sirens, naiads, or sphinxes might guide them into the ward.

Tizerus

The deepest ward of the Underworld, Tizerus is the final punishment of murderers and those who committed unforgivable offenses against the gods themselves.
The ward is a gloomy, miserable expanse where the ruins of titanic architecture loom over a wasteland of black rock. At its centre looms Erebos’s palace, an ominous structure of polished darkness embellished with gold ornaments and streamers of crimson silk. The god of the Underworld broods in his halls, occasionally striking out with his lash to drag wayward souls into his realm. Deep beneath the palace lies the lone glimmer of hope amid the ward’s gloom: the portal called Lathos, which leads back to the world of the living. Beyond Erebos’s palace sprawls a stinking swamp of rotting muck and dead vegetation known as the Mire of Punishment. Souls trapped there suffer eternal torment at the claws of demons and other monstrous creatures.
When souls damned to Tizerus arrive, they quickly find themselves dragged away in a whirlwind of pain and terror. Demons, Underworld harpies, lamias, and nightmares compete for the chance to haul the doomed away to their own personal punishments.