Mount Doom is a fictional volcano in J.R.R. Tolkien's Middle-earth legendarium. It is located in the northwest of the Black Land of Mordor and close to Barad-dûr. Alternative names, in Tolkien's invented language of Sindarin, include Orodruin ("fiery mountain") and Amon Amarth ("mountain of fate").
Mount Doom didn't just appeared in Lord of the Rings
It also appeared in the Legendarium as the statement says