Amenti, the Dark Kingdom of Sand, is the Dark Kingdom of the Undying in the Underworld, as well as the final resting place of Osiris. It's not populated by wraiths, but by Immortals, who died and found themselves back in the Shadowlands.
As the earliest human society arose in ancient Egypt, guided by the vampires Set and Osiris, the awakened individuals came to understand that the dead had not completely abandoned the world, simply retreated to this new Underworld, which they called the Duat. To mages like Isis, the Duat was not the great mystery then that it is now and when the first Immortal was given the Spell of Life he did not go into the afterlife unarmed. The Hekau paths of Necromancy and Ushabti allowed the first Immortals to expand and fortify the realms they found in the Duat. Echoes of the living lands formed by the passions and desires placed upon the great cities, tombs and temples of the Kingdom. The substance of the Duat was workable by the Immortals and they built their shadow realm Amenti into a stunning kingdom beyond the shroud. In later years, the Shemsu-Heru and the Ishmaelites taught the necessary skills to the other Dynasties, allowing each of them to found shadow realms of their own.
Charon was, to the Undying, quite a latecomer. His empire of Stygia, built during the time of the Roman republic, a comparatively modern event. Amenti stood in the Underworld for thousands of years before the structure of the Dark Kingdom of Iron was even begun and its wraiths can be forgiven if they barely pay the slightest mind to the Hierarchy, the Guilds or the Legions. The same is true of all the Shadow realms built by the other Immortals, or more correctly, by the legions of Ushabti in their employ.
But as the Dark Kingdoms did begin to arise some of them pushed the Shadow Realms into the shadows, either outstripping by dint of sheer numbers or attempting to conquer them out of a desire for their power and riches. Erlik Khan had ruled the Underworld of the east from Siberia to the Yangze River for centuries but it was toppled by Qin Shi Huang (Yu Huang). Amenti, with the aid of great magics, slipped from sight.
It remained independent and protected from the stygian empire because of the power of their dead and by the treaties forged between Anubis and the Ferrymen. To this day, the Shadow realms remain masked from anybody not invited to journey there, hidden by power spells of stealth and misdirection and by the untiring Ushabti guardsmen.
Today the Underworld is less about geography and more about concepts and emotions in locations only partially connected in a recognisable fashion. The dark kingdoms dominated the Shadowlands for two thousand years, with the Shadow Realms hidden away on the edges of the tempest, but now the old order has fallen and the Duat is in tatters.