Old One is a term for a deity or other ancient, powerful supernatural entity. This term is often used in fiction, primarily in fantasy and horror fiction.
Throughout the weird fiction of H. P. Lovecraft, the term "Old Ones" is employed in various contexts. His first mention of the Old Ones appears in "The Call of Cthulhu" (1926), where he uses the term in reference to a group of primordial beings entombed in the mythical city of R'lyeh. At one point in the story, Inspector John Legrasse of the New Orleans police department raids a cult ritual gathering, capturing several of its members:
They worshipped, so they said, the Great Old Ones who lived ages before there were any men, and who came to the young world out of the sky. Those Old Ones were gone now, inside the earth and under the sea; but their dead bodies had told their secrets in dreams to the first men, who formed a cult which had never died.
Lovecraft also mentioned the Old Ones in "The Dunwich Horror" (1929), naming them as mysterious entities associated with the Outer God Yog-Sothoth. In The Shadow Over Innsmouth (1936), the Old Ones had the power to keep the Deep Ones in check. In Lovecraft's revision story "The Mound" (1940), the denizens of K'n-yan are referred to as "Old Ones".
In Lovecraft's novella At the Mountains of Madness (1936), "Old Ones" was another name for a fictional alien species, the Elder Things. These creatures were said to have built cities around the world in ancient times, but were eventually relegated to Antarctica. At the end of their reign, they were all but destroyed by the shoggoths, a slave race of their own creation.