Lord of Balrogs was the title of Gothmog. He was a servant of Morgoth and High Captain of Angband during the First Age and probably led the group that saved Morgoth from Ungoliant in Ered Wethrin. In the Dagor-nuin-Giliath (Battle-under-Stars) he mortally wounded Fëanor.
In the Nírnaeth Arnoediad (Battle of Unnumbered Tears) he killed Fingon, High King of the Ñoldor, with his axe and a little help from another Balrog. In the Fall of Gondolin Gothmog was finally killed by Ecthelion, who was mortally wounded in the process.
His successor among the Balrogs was Durin's Bane, an extremely powerful Balrog, probably almost as powerful as Gothmog, as he was able to expel the Dwarves from Moria and was even able to cast a counterspell on Gandalf, almost killing him. He gained his name because he was the one who slew Durin. Durin's Bane was finally slain by Gandalf, who was travelling with The Fellowship of the Ring. Gandalf knew that Durin's Bane was far too strong for the others, so he challenged it in a single battle, in which they both died but Gandalf was returned to life by the Valar and Eru Ilúvatar.