(Plane of Earth)
Source: The Emerald Spire Superdungeon, pg(s). 158
Xiomorns are a race of primeval elemental outsiders from the Plane of Earth, best known in modern Golarion as the Vault Keepers. Among the oldest known life forms, xiomorns travel to young planets in the Material Plane to conduct experiments into the nature of life and evolution, though they have little interest in the indigenous lifeforms of these worlds aside from their roles in the xiomorns' experiments. Planets visited by xiomorns feature strange ruins and great vaults that hold strange ecosystems, such as the vaults of Orv within Golarion's Darklands.
Both castes of xiomorns are towering, four-armed creatures made from stone, with an insectile head, thorax and upper limbs. The Vault Builders are larger and have four legs, whereas the Vault Keepers are smaller and only have two legs.
While much of the history of the xiomorns has been lost to Golarion's surface dwellers, xiomorns were created at the dawn of time by the elemental lord Sairazul with the help of a more powerful deity or some primal urge of the Plane of Earth itself. Their insectile form was based off Sairazul's own image, and she created exactly 65,536 xiomorns after the perfect number of facets of a supernaturally sublime jewel she found in the Plane of Earth.
Sairazul was later imprisoned and defeated by the evil elemental lords, causing most xiomorns to be unaware of her role in their creation and assume that they were spontaneously created. When they first encountered Sairazul's evil rival Ayrzul, they had no reason to fear or distrust him. Ayrzul showed the xiomorns a vision of their race going extinct during a mental invasion of the yithians, who swapped their minds and bodies with the xiomorns, leaving them to be destroyed by the cataclysm that should be killing the yithians instead. The xiomorns knew despair for the first time. Seeking the mythic essence that Sairazul imbued into the xiomorns, Ayrzul further asked them to sacrifice a quarter of their number so he could have the power to teach the rest how to forestall their demise. The xiomorns were appalled by this suggestion but still believed Ayrzul and in the end they decided to sacrifice the mythic essence of half their number instead, granting Ayrzul the same influx of primordial power without having to destroy any of their own kind. Those that lost their mythic essences became the non-mythic Vault Keepers, while the others who retained theirs are called Vault Builders.
In exchange for their essence, Ayrzul gave the xiomorns knowledge of the Vault Seeds, capable of creating an immense underground cavern and infusing it with a magically supported ecosystem. In order to stave off their foretold doom, the Vault Builders used the Vault Seeds to create vaults that supported particular kinds of life in the Plane of Earth, while the Vault Keepers were tasked with cultivating them so they would not go awry. When they realised that the Plane of Earth was too limited for the ecosystems and life-forms needed to save their race, they opened portals to various worlds in the Material Plane, where the diversity of life and habitat is greater.
The pechs were one of the first xiomorn creations on the Material Plane, made from benign fey earth spirits, but were quickly determined to not hold the secret of the xiomorns' salvation. The Vault Builders, before moving on, assigned them to the Vault Keepers to compensate for their sacrifice. Many pechs continue to live as the Vault Keepers' slaves, while others escaped and became deros.
For countless years, the Vault Builders created self-contained and magically sustained ecosystems across the universe where they can support the myriad forms of life and conditions with which they experiment. After a few millennia, the xiomorns would inevitably end their experiments as failures and move on. They refuse to divulge the details of their plans, for fear of alerting the yithians.
The xiomorns played a huge role in the early history of Golarion, especially the Darklands. Though they shared similar interests with the alghollthus, the two races differed in ideologies, and their primordial empires turned to war, a war for which xiomorns are more well-known among Golarion's surface dwellers. As the cost of this war proved too much, the xiomorns decided that it was lost and most of their race fled Golarion. The alghollthus watched the xiomorns' departure with amusement, knowing that the biological salvation sought by the xiomorns lay within the capabilities of the alghollthus themselves.  Few xiomorns remained, usually deserters or lone Vault Keepers who kept tending their posts after the departure of the rest of their race, and all that remains of their influence is the Vault of Orv, buried deep and far away from most of Golarion's inhabitants, and the enigmatic ruins and inscriptions from their bygone era.
Xiomorns have no biological sex, do not reproduce, and naturally live for thousands of years. When a Vault Builder is approaching its natural death, it will form a crystalline Generation Stone which can be retrieved and stored indefinitely while the body decays, and holds all memories of the dead. Vault Builders can magically build Vault Builder bodies, and putting a Generation Stone inside it will resurrect the dead Vault Builder: because of this, if its remains are recovered and properly stored, a Vault Builder can live forever. If a Vault Builder is killed, the Generation Stone will not form and it remains permanently dead.
Vault Keepers live for only a couple of thousand years and can neither form Generation Stones nor be resurrected; instead, Vault Builders can use Vault Seeds to create entirely new Vault Keepers. The number of Vault Keepers can never exceed 32,768: it is impossible to create more.
Xiomorns were divided into two castes: the Vault Builders and the Vault Keepers. The mythic Vault Builders were masters of crystalline magic, able to create and shape both life and terrain with their minds. They ruled over their lesser kin, the Vault Keepers, who were tasked with maintaining and protecting xiomorn experiments.
Only the Vault Builders could use the power of the Vault Seeds to create new vaults. Vault Keepers who tried to tap into the seeds' power were unable to control the magic, creating spectacular backfires such as the one that created the Emerald Spire in the northwestern River Kingdoms.
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