Ymir
On the cradle of creation when the divine spirits were sculpting the cosmos, the primals came to be, living manifestations formed of the great swell of magic that flowed through the many branches of the lifestream. There was Caelum of the raging storms, Gaia of the forest, Anukiss of the oceans, and so on. At the heart of the astral planes and beyond the corporal world, Ymir dwelt, seeing and feeling all, a cosmic entity that was unlike any other save for perhaps the overseer itself.
When humanity was still gaining its bearings during the beginnings of the world, the chaotic nature of many of the primals were something to fear. A sudden earthquake, tsunami, or storm of catalytic levels was quite frequent, as the world was still young and magic was indiscriminate, and so were these greater entities. But the men and women of the world were by no means helpless, and when faced with such adversity it was survival that was the priority, developing their own means of combating the threat as magical theory developed and both weapons and tools emerged.
Over time, these primals had children of their own, the primordials. More prone to reason and sophisticated in their own ways. While the details of the events are unclear, lost to the many millennia that has since passed, this era was known as the Primomachy. An alliance of humans and primordials against the primals, and over time the former were victorious, escaping extinction and thusly establishing the foundations of a world where civilization could thrive.
Before Astya, before Brittania, Gehenna and Valmasia, Sheng and Agartha, Esshar, and Meranthe, these sacrifices occurred during the mythical era. Arguably the most notable of all was when Ymir split into ten separate Aspects, establishing the greatest and most influential pantheon of gods, formed to change the tides of a losing war, and granting them the means to defeat the out of control primals. These ten, led by Enarr, went on to found the realms of the netherworld, imprisoning and banishing wild spirits as well as forming the natural separation we know today.