J.R.R. Tolkien calls the writing system used for the One Ring inscription Tengwar.
Tengwar is a script devised by Tolkien for several of his invented languages. The Ring inscription is written in Tengwar characters, although the words themselves are in the Black Speech of Mordor. The inscription begins, “Ash nazg durbatulûk,” meaning “One Ring to rule them all.”
The script’s name comes from Quenya and is commonly translated as “letters” or “characters.” Tolkien designed it as a flexible system in which signs represent consonants and vowels according to different modes. Its elegant forms are among the most recognizable visual features of Tolkien’s writing.
Tengwar is distinct from Cirth, another writing system in Tolkien’s legendarium. Cirth consists chiefly of angular runic signs and is associated especially with dwarves. The Ring’s fiery inscription is therefore not written in ordinary runes, even though it can be mistaken for a generic fantasy alphabet.