Old One

An Old One is a species of incredibly powerful Immortal that were originally personified in the first and only ever born Immortal: Silas, Esther's eldest son Aedan Mikaelson attempted to join with Silas by performing Expression magic through an Expression Triangle and became an Old One himself, Aedan formed Silas's Cult however his family imprisoned him and brought his confined state to the New World with them sealing the knowledge of creating Old Ones with him.

Trivia

 * The Old Ones are the primordial Vampires in the Vampire Diaries Novels.