Sauron’s fictional language used for the One Ring inscription is called the Black Speech.
The inscription on the Ring begins, “Ash nazg durbatulûk,” and is written in the Black Speech of Mordor. Its complete text expresses the Ring’s power to find, rule, and bring all the other Rings under one authority.
Tolkien created the Black Speech for Sauron and his servants, but it is comparatively sparsely documented beside languages such as Quenya and Sindarin. The language’s harsh sound and limited surviving vocabulary fit its association with Mordor and domination.
The inscription is sometimes mistakenly identified as Elvish because it appears in the Tengwar writing system. Tengwar is a script, not a language. Tolkien used that script to write words from different languages, including the Black Speech inscription.