The Great Ancients are extraterrestrial creatures from the cosmos that evolved upon a planet within the infinite expanse of Yggdrasil; They worship the Outer beings that exist in the abstract dimensions above them. They sail the stars on their ships of cosmic dust and formed matter. Millennia ago, they were sent to the planet Earth and spread their vast wings to conquer the lands and worship their gods. They built the city of Uul’Khaem (Ool-Khay-em) upon the earthly continent of Ool’. The Ancients landed upon Ool’ many eons ago, where the gods of Earth resided along with two other species e.g. the Atlanteans (Yes, those Atlanteans) and the Star Watchers. The Great Ancients arrived upon the Earth to commit sacrifices for their Gods — they firstly attacked the Star Watchers, who were the most advanced lifeform upon the young Earth — this would prove incredibly difficult for the Great Ancients as the Star Watchers and their magical energies were much too prevalent as powerful for the Great Ancients to overcome, and so, they set their sights upon the Atlanteans and the city of Atlantis. The Atlanteans were not nearly as in depth with mystic abilities and technologies — The Great Ancients overcame the Atlanteans within two fortnights and the city sank beneath the waves of the young oceans. This act deeply angered the gods of Earth including the Egyptian Ra and the other Olympian Gods Poseidon, Zeus, and Hades. Within a single strike the Gods of Earth decimated the continent of Ool’ and sent the city of Uul’Khaem beneath the waves. This attack killed many of the Great Ancients to where they communed with their deity Malth’Karis to save them from their grave, and he set an ancient spell upon them that would cause them to hibernate for many millennia. The Great Ancients now sleep at the very bottom of Earth's oceans, and they are set to challenge the Gods of Earth for many more Eons. They are not restricted by biological means, and can morph their bodies into whatever form they desire; The most commonly depicted form of the Great Ancients is an oblong torso with two legs and two arms; the Great Ancient face is vaguely cephalopodian and they have great leathery wings upon their back that extend and fuel from the grotesquely pulpous torso. 

The Star Watchers are gelatinous shell-fish shaped beings from the stars resembling fleshly plants; They arrived on the newly formed Earth eons ago when the crust still bubbled with magma and fire — The Star Watchers have three upward pupils upon long eyestalks that fringe and twirl with every movement from their gyrating torsos — They are the progenitors of all life upon the Earth and dominated the surface long before the Gods of Earth were there; Another name for them is the Star People due to their hexagonal bodies resembling the shape of a star — The Star Watchers bodies are a thick as tree trunks and they carry themselves upon ten writhing tentacles fifteen feet in length, alongside their pulpous bodies at five feet in height. The origin planet of the Star Watchers is unknown and rudimentarily mentioned between them with their abstract language. The Star Watchers formed an alliance with the Gods of Earth once the planet had cooled over the course of untold billions and they shared land with the Atlanteans and the Olympian Pantheon. The Star Watchers with their magical technology and unbounded abilities were much too difficult for the Great Ancients to defeat and so a truce was made between the old aliens and the Star watchers witnessed the fall of Atlantis and as the Atlanteans were cruelly enslaved by the Great Ancients for sacrifices. The Star Watches would also witness the Great Ancients' downfall as the Olympian God Zeus, Hades, and Poseidon alongside the Egyptian Deity Ra struck down the continent of Ool’ and the city of Uul’Khaem. This caused the Outer Monster Malth’Karis to inform the other Outside Beings of Ool’s sinking and the destruction of Uul’Khaem, onto which the enslaved deity Zeph'naloth whispered to Azazel The Great Devourer who was deeply angered by the destruction of Ool’ and with a single motion he sent his white flame down upon the young planet and obliterated the population of Star Watchers and broke apart the continents sending Olympus beneath the Earth's land.

(Context in the comments)

The Great Ancients and The Star Watchers (Mosaic Mythos)