The Sidar were not always immortal. They began as another band of hardy men and women with the fortitude and ingenuity to survive both the ice and the beasts which its melting unleashed. As with all such groups their origins are submerged in the bardic legends that substitute for history in an era alienated from the written word. All we know for sure is that they found a way to survive in a land by a frozen sea surrounded by endless snow-laden woods.
These prehistoric Sidar moved from place to place, coaxing from the thawing earth where they could, and slaying beasts where they could not. The beginnings of the Rebirth did not so much ease their lives as gradually lessen its crushing burden. Their world was one of unyielding frozen soils, an endless wilderness of towering snow-laden pines, eternally frozen in the Age of Ice, slowly rekindling in the brief summers of the early Rebirth.
Their future capital, Celo, was originally a sacred burial space for the Age of Ice nomads of the region. The sea, then an endless desert of ice, was believed to be the dwelling place of Arawn, god of death. They would carry their dead as far as they dared through the ice and then leave them there to dwell with Arawn. Great stones which straddled the coast seemed to the Sidar to be gateways to the land of death. In the corrupted Patrian of the ancient Sidar, 'Celo' meant something like "gateway to death". Here many nomad groups of the region would gather to pass through the gates and deliver their dead to Arawn. It was this practice which differentiated the nomads from other wanderers of the region. The Sidar were simply the nomads who visited these gates of death and shared in the rituals, remembrances and rites which surrounded it.
When Erebus finally experienced its first faint summer after Mulcarn's destruction, the Sidar considered the waters which melted into Celo as holy and were inspired to develop a preceding stage to the long journey into Arawn's land. The seasonal melting of the sea spoke to them of the connection between the world of the dead and the world of the living. They dug out hollows in the earth around Celo and channels leading to the hollows. The dead were placed in the hollows and, when it melted, the sea flowed down the channels to wash over the waiting corpses, cleansing them in preparation for their journey to the afterlife. This journey now had to wait until the deepest Winter, when ice covered the sea once more.
This mean that the funeral of an ancient Sidar who died in late Summer would not be completed for nearly two years. First, they would be preserved in ice until next year's Summer when they could receive the necessary bathing in the melted seawater. Then they would be stored in ice again until deepest winter when they could be carried into the frozen ocean.
This caused Celo to become a place which could not simply be abandoned. The dead stored there needed to be guarded from marauding beasts. The sacred hollows and channels needed to be guarded from desecration by non-Sidar. The nomadic Sidar tribes began to co-operate to ensure the protection of the site and share the burden of maintaining its permanent settlement of priests and guards. This was the foundation of Celo-as-a-city, a tiny enclave of funerary-priests of Arawn and their devoted guards.
These priests were so tightly bound by ritual that they had no need of a formal hierarchy of leadership. They would do the duties of the funerary rites and preside over the annual remaking of the hollows and channels, the preservation of corpses and digging of the pit in the ocean. Collective decisions were made by the gatherings of tribal chiefs, as they decided how best to preserve Celo.
- A History of Erebus, Morrogoth the Amber