Dûrûr (literally meaning "Dark Flame" in Sindarin) was a Balrog of Morgoth who's rank at that time was lost. He claimed he was one of the first Maiar to join the Melkor. Dûrûr most likely fought in every Battle of the First Age, and was most likely overwhelmingly loyal to Morgoth Bauglir as were the other Balrogs.
This Balrog of Morgoth fled from Middle-Earth along with an army of Orcs after their Master was defeated and came upon the place Men called the Dark Land sometime during the Second Age after the War of Wrath and Destruction of Beleriand. He continued inland and fell into a slumber from exhaustion while the Orcs discovered a plant that granted unnatural long life.
During the Second Age, Ar-Pharazôn and other Numenorian Kings and Queens sent ships down to the Dark Land for colonization and exploration. But after the fateful Destruction of Numenor, the colonies lost contact with Men. Instead of dying off, though, these Men and Women survived and retained the knowlegde of their ancestor's from the North. New Numenor they called the land. They smithed, farmed, read, wrote, and so on. But these Men were also arrogant, for they explored deeper and deeper inland, not heeding to what dangers might lay in wait. Eventually, they awakened Dûrûr and ran in fear from him and his horde of Orcs. One man named Naktura led a rebellion against the Balrog and thus started the Dagor DurTaleth (Battle of the Dark Land).
Dûrûr succeeded in killing Naktura and driving the Men into hiding. For years, the Balrog roamed across the land and proclaimed himself Hirren DurTaleth (Lord of the Dark Land). With help from his Orcs, who submitted to his rule, he built cities, towers, strongholds, bridges, mines, and so forth. After years of construction, Dûrûr built a city-stronghold Broniad (meaning "to endure"), in which he dwelled.
But in year 102 FoA, the descendant of Naktura, Belgion, lead yet another rebellion against Dûrûr starting the War to Reunite, or Dur Aderthad. They fought for 5 years before Belgion and his Army laid seige to Broniad. For three years they fought, and then Dûrûr led an army of 13,000 orcs against the army of Men around 15,000. The whole Orc Army clashed with the Army of Men for five days and four nights before Belgion slayed Dûrûr after the Balrog killed Kildirac, a lifelong friend of Belgion and Captain of the Archers. The magority of the orcs were killed there but the ones that were able to flee were hunted down and destroyed.