The inscription on the One Ring in J.R.R. Tolkien’s works is written in the Tengwar script.
The fiery inscription is the Ring-verse, beginning “Ash nazg durbatulûk,” and is written using Tengwar characters. In the story, the letters become visible when the Ring is heated, revealing the writing associated with Sauron and the power of the One Ring.
Tengwar is a writing system rather than a single language. Tolkien used it for several languages, including Quenya and Sindarin. Cirth is another writing system in the legendarium, consisting of angular runes, while Sarati is an earlier script associated with the fictional history of writing in Middle-earth.