Which fictional language contains the One Ring inscription in J.R.R. Tolkien’s legendarium?

The One Ring inscription in J.R.R. Tolkien’s legendarium is written in the Black Speech of Mordor.

The inscription begins, “Ash nazg durbatulûk,” translated as “One Ring to rule them all.” Sauron created the Black Speech for the servants of Mordor, and the language appears most prominently in the Ring’s verse. Tolkien wrote the inscription in the Tengwar script, but the script and the language are different things.

This distinction causes a common mix-up. Tengwar is a writing system, while Black Speech is the language used for the inscription. Quenya and Sindarin are Elvish languages, and Khuzdul is associated with the Dwarves. The Ring verse also includes lines referring to the Elven rings, the Dwarven rings, and the Nine rings given to mortal Men.