Lungorthin was the Lord of the Balrogs in an early stage of The Silmarillion. He is not mentioned in the later Silmarillion, or the published Simarillion.
He is mentioned only once, in The Lay of the Children of Húrin; after Húrin was captured by Morgoth, Lungorthin smote him on the mouth.
J.R.R. Tolkien, Christopher Tolkien (ed.), The Lays of Beleriand, "I. The Lay of the Children of Húrin", pp. 98, 102-3