What is the fictional language of Mordor used for the One Ring inscription?

The fictional language of Mordor used for the One Ring inscription is the Black Speech.

Sauron created the Black Speech for the servants of Mordor, and the Ring inscription is its best-known surviving example. The inscription begins with “Ash nazg durbatulûk,” translated as “One Ring to rule them all.”

Tolkien wrote the verse in the Tengwar writing system, but the script and the language are different things. Tengwar is a writing system used for several languages, whereas Black Speech is the language spoken in the inscription.

Black Speech is often confused with Mordor’s other languages or with Elvish tongues such as Quenya and Sindarin. The Ring verse’s English translation appears in The Fellowship of the Ring and connects the language directly with Sauron’s power.