Which language does J.R.R. Tolkien use for the inscription on the One Ring?

J.R.R. Tolkien uses the Black Speech for the inscription on the One Ring. The fiery writing gives the Ring’s verse in the language created for Sauron and Mordor.

The inscription begins, “Ash nazg durbatulûk,” translated as “One Ring to rule them all.” The complete verse describes the Ring’s power over the other Rings of Power and its role in binding their bearers to Sauron’s design.

Tolkien created the Black Speech as a harsh language associated with Sauron. It is distinct from Quenya, a refined Elvish language, Sindarin, another major Elvish language, and Khuzdul, the secret language of the Dwarves. In the story, the writing becomes visible when the Ring is heated.