The fictional writing system used for the One Ring inscription is Tengwar.
Tengwar is a writing system devised by the Elf Fëanor in Tolkien’s mythology. Its characters, called tengwar, are used for several languages of Middle-earth and can represent different sounds depending on the mode being used. The script is therefore a writing system rather than one single language.
The inscription on the One Ring is written in Tengwar, but its words are in the Black Speech of Mordor. The famous lines begin, “Ash nazg durbatulûk,” meaning “One Ring to rule them all” in the translation presented by Tolkien. This distinction between script and language is an important detail in reading Tolkien’s invented writing systems.
Tengwar is sometimes confused with Cirth, another Tolkien script whose characters are angular and rune-like. Sarati is an earlier writing system created by Rúmil, while Angerthas refers to expanded runic arrangements related to the Cirth. These systems belong to different stages or traditions within Tolkien’s fictional history.