A brief explanation on origins of Celestials in marvel Multiverse
This one eyed creature is called THE FIRST FIRMAMENT. Many eons ago, it was the very first Eternity. It was all there was in existence, and became lonely. So it created the Aspirants. Some dark in appearance, while others were multi-colored. The Aspirants worshipped the First Firmament, which pleased him (it). However, the multi-colored Aspirants rebelled against their darker brother’s. War broke out, tearing the Firmament into pieces. Most of the pieces held together and fled. The torn pieces formed another Eternity, in the void that the Firmament left behind. Time passed, and many Eternities formed. The multicolored Aspirants won their war over the dark ones, and with each new Eternity, it would subconsciously continue to create the multicolored Aspirants. The multi-colored Aspirants became the Celestials, the dark ones were now called Dark Celestials. Inside Eternity, the Celestials were given purpose, creating life on different planets, shaping the Universe, which pleased Eternity