The lost city of Valoria

Valoria was once the mighty capital city of the Human empire, yet now only ruins and rubble remain. During The War for Eternity, the human empire was at it's peak. Valoria was its shining jewel, a marvel of its time. The Human Empire revered many gods, yet, to them, one stood above all others. Thanatos, at the time god of Beauty, Ambition and Youth, embodied their ideals most thoroughly. Humanity, in its successes uniting the various clans across the globe, had grown ever-more ambitious. With the belief in Thanatos growing ever-more prevelant, he grew in power as well. Thanatos had become discontent at his place beside the council, and plotted to take the place of one of the major gods. Yet not just any seat on the council would be good enough for Thanatos, for he set his sights on X, the then-god of death. To do this, Thanatos would require allies. He found those in Aethereos and Solaris. An unlikely alliance was formed. One of Solaris' creations, the Devils, had long wanted to plunder X's vault of souls. Soul Coins had long been one of the most valuable currencies in all the realms, and while occasional skirmishes into the vault could plunder a few at the time, the Devils smelled their chance, and convinced Solaris that X had grown too powerful, and should be shown his place as an equal on the council. Aethereos' reason for pledging his help was more idealistic. Long had he sought to give mortals true purpose. He had given them an incredibly complex world, with riddles and arcana, yet ultimately, there was nothing waiting for them beyond death. Just a dusty vault. Aethereos wanted to see this changed. He wanted mortals to have an afterlife that made their achievements in life worth the effort. Solaris pledged the help of his Devils and Tieflings, while Aethereos pledged the help of a mortal-avian hybrid species he had created, called Seraphs[Valkyries]. Together, the mortals, devils, and seraphs would fight against the forces of X. The war was long and arguous, and while Devils and Seraphs could not possibly be more different, their shaky alliance held strong for a while. Many mortals died, and near the end of the war, when X was finally defeated, the Devils turned on the Seraphs, hoping they could loot the Great Vault of Souls themselves. Solaris was, as he is quite often, too distracted to keep his followers in line. While they bickered over who would claim the afterlife, the city of Valoria was slaughtered in its entirity by X's Aether Wraiths. Humanity's gods were too distracted to hear their pleas for help. As Valoria fell, so too did the human empire, from then on out splinitered as separate warring kingdoms and clans. Ultimately, Thanatos and Aethereos were victorious against Solaris' devils. Thanatos claimed from X the domains of Darkness and Death, while retaining his aspect of Beauty. Aethereos helped him set up a new afterlife. Instead of X's Aether Wraiths, Aethereos' Seraphs[Valkyries] would now bring the recently deceased to the realm of the dead. Yet instead of a dusty vault, they would now find themselves in an Endless Desert filled with Aethereos's Sands of Time. His gift to Thanatos for helping rid the world of X's meaninglessness. This new afterlife would be one based on reflection on one's achievements in life. This was Thanatos' reward to the mortals that fell fighting on his side. They would get to relive their victories for all eternity. So ended the time of man, and so began the time of the Elves.
As a result of the way the war ended, the remaining humans now despise Devils and idolize Seraphin. Across the world, it's possible to find shrines made in the memory of Seraphin who fell defending human mortals from Aether Wraiths.
More Recentlly, The White Wizards and their order has been sending scouts into the ruins of Valoria to look for long-lost relics, and see if re-colonization might be a possibility. Yet their scouts have been meeting some resistance, with reports of slithering creatures in the waters making the work very dangerous and near-impossible.
The players discover a knight of the white order who was slain by a tentacle creature. He carried a puzzlebox, that works kind of like a compass, pointing them towards a shrine. The shrine has statues of each of the leaders of each of the human clans that made up the ancient human empire. Each depicts their leader in a pose reminiscent of their culture, one is thoughtful, one looks resolute, and some of the statues are partially or entirely destroyed. At the base of each of the statues is a groove that seems like it was made for a gemstone. The gemstones are missing, but clues indicate that each of the leaders used to carry a gemstone that would activate some type of mechanism. If the party seeks out the gemstones, the puzzle box will activate once they get near to a gemstone, reacting to its energy. Should they let the gemstone touch the puzzlebox, it shoots out a blast of energy, providing all nearby with a vision of how the holder of the gemstone, the last leader of their clan, died. If they bring all of the gemstones back to Valoria, they open a gateway across the realms that was once used for the empire to wage war against the realm of the Dead. Now, the party may use it to travel the realms for their own purposes.