R'llyeh was built by hideous otherworldy ancient beings from the stars. Who ruled the Earth ages ago, building great cities and working their magic in unfathomable ways. Their servants were kept chained, living and dying in the dark glory of their masters. On the walls of the city, the magic of these ancient beings were carved into writings.
Which are theorized as being the source of all magic on Earth, which was imperfectly copied into the Darkhold and through it the Necronomicon. The sinking of Atlantis and the return of man to the sea awoke the ancient evil. Who used the formless substance called "Sh'g'th" to create unholy life, the first Aqueos (a race of aquatic vampires).
Relationship to the R'lyeh of CthulhuEdit
In the Marvel prime reality, both R'lyeh (the city of Cthulhu) and R'llyeh exist as separate unique locations. The former is tied to the Old Ones and it was invoked in the incantations of the worshipers of Set and the Old Ones during the Hyborian Age, and during the late 20th century an eldritch abomination from R'lyeh was encountered by Doctor Strange. The latter in turn is tied to the book of chaos magic, the Darkhold which was created by Chthon an Old One and in the Marvel universe is the source of the Necronomicon.
It's possible that may change in the future. As the lore of Marvel is so vast and convoluted, it's possible that R'llyeh is actually the same R'lyeh spoken of during the Hyborian Age. In which case the "Ancient Things" are actually the Old Ones. The discrepancy is also evident in the origin of the Darkhold. With lore stating it was created by Chthon, billions of years ago when the Elder Gods walked the Earth. Yet, R'llyeh is also stated to be the source of the Darkhold. Therefore somewhere down the line, it's likely that Marvel will either retcon the Darkhold's origins, erase / ignore what was written in "Namor: The First Mutant", or merge them both.