The Lost World Novels: Dwellers in the Mirage & The Moon Pool

"The Moon Pool" deals with an advanced race which has developed within the Earth's core. Eventually their most intelligent members create an offspring. This created entity encompasses both great good and great evil, but it slowly turns away from its creators and towards evil. The entity is called either the Dweller or the Shining One.
"Dwellers in the Mirage" Leif Langdon is an American explorer who discovers a warm valley in Alaska. Two races inhabit the valley, the Little People and a branch of an ancient Mongolian race; they worship the evil Kraken named Khalk'ru which they summon from another dimension to offer human sacrifice. The inhabitants recognize Langdon as the reincarnation of their long dead hero, Dwayanu. Dwayanu's spirit possesses Langdon and he starts a war with the Little People.
Civilization

Abraham Grace Merritt (1884-1943) was an American Sunday magazine editor and a writer of fantastic fiction. The Science Fiction and Fantasy Hall of Fame inducted him in 1999, its fourth class. Merritt's novels and stories typically revolve around conventional pulp magazine themes: lost civilizations, hideous monsters, etc. His heroes are usually gallant Scandinavians, his villains treacherous Germans or Russians and his heroines often virginal, mysterious and scantily clad.