The words beginning “Ash nazg durbatulûk” in Tolkien’s One Ring inscription are written in the Black Speech of Mordor. Sauron created the Black Speech for the servants of Mordor, and the Ring’s inscription presents the most famous surviving example of the language.
The complete inscription is a two-line verse describing one Ring ruling the others and binding them in darkness. It is written using Tengwar letters, but the script and the language are different things: Tengwar is the writing system, while Black Speech is the language being represented.
Readers sometimes identify the inscription as Elvish because it uses Tengwar, a script strongly associated with the Elves. That is a category mistake. The Ring poem is in Black Speech, and Tolkien gives its English translation in The Lord of the Rings. The language is deliberately harsh in sound and closely tied to Sauron’s power.