The balrogs are
Tolkien invented the name "Balrog", providing an in-universe etymology for it as a word in his invented Sindarin language. He may have gained the idea of a fire demon from his philological study of the Old English word Sigelwara, which he studied in detail in the 1930s.
No, Balrogs do not serve Sauron. Similar to Sauron, Balrogs are Maiar, powerful spiritual beings second only to the Valar themselves. They were corrupted by Melkor's evil before the first age and were made into Valaraukar (Demons of Power).
They are humanoid, and very intelligent creatures (being Ainur, they took part in the creation of the world). In the books, Tolkien depicts the Balrog as being barely a shape wreathed in shadow and flame.
Lord of Balrogs
The most powerful of all the Balrogs, one of the chief servants of Melkor, who held an authority hardly less than Sauron himself. A wily commander and fearsome fighter, Gothmog was often accompanied by others of his fiery kind, and at least in the Nirnaeth he had a personal guard of dozens of Trolls.
Famed Balrogs include Gothmog, slain by Ecthelion, and Durin's Bane, slain by Gandalf.
Knocking Ecthelion's sword out of his hands, Gothmog prepared to strike at him, but, as he raised his axe, Ecthelion ran forward at the Balrog lord, ramming his pointed helmet into the beast's chest and forcing both of them into the Fountain of the King.
So, one glimpse into Smaug's eyes and the Balrog falls under the spell. Even if it's just for a second—a moment of hesitation or distraction, it'd be enough. Smaug would snatch up Durin's Bane and gobble him up with his sword-sharp teeth (and we know swords can kill Balrogs).
The Balrog met the challenge and attempted to strike the Wizard with its fire sword. But this fell blade was broken by Gandalf's power. The Balrog became wary and angry in turn.
I was just curious if Durin's Bane had taken The Ring from The Fellowship could they have used it? Yes, anyone can use the Ring.
Peter Jackson certainly makes the assertion in the theatrical and extended editions of the Hobbit movies that Smaug and Sauron are not only well aware of each other, but are planning to ally with each other. Smaug even seems to have detailed knowledge of Sauron's coming plans.
Many audience members know that the Balrog was awakened because the Dwarves delved too deep in Moria. Their greed for Mithril drove them deeper and deeper into the earth until the Balrog woke and slaughtered them all.
For more than five millennia, the Balrog hibernated in his deep hiding place at the roots of the mountains in Khazad-dûm. He remained undisturbed throughout the Second Age and most of the Third, before the mithril-miners of dwarf-King Durin VI awoke him in T.A. 1980.
Balrogs generally took the form of tall, menacing beings roughly humanoid in shape, though seeming to consist of or be surrounded by shadow and flame. They used both a flaming sword, and a fiery whip; they were constantly burning, and their weapons appeared molten.
However, in the Tolkien novel, Gandalf actually never knew about the Balrog. All he knew was that Moria had been evacuated because of something called Durin's Bane, and that it still lurks inside. And while he does know that Orcs attacked the Dwarven kingdom, it was many years ago, and so he hoped they were gone.
The Eagles came to help them, but Glorfindel fought the Balrog alone upon the pinnacle of a rock. Both died after falling in the abyss, and their duel is sung of by many songs.
Third Age. For more than five thousand years, the Balrog lay dormant at the roots of the Misty Mountains beneath the Dwarven kingdom of Khazad-dûm.
A Balrog is definitely stronger than the witch king.
Gandalf was scared of this beast because he knew how powerful it was since it was a Maia like himself, but it was purely evil.
Sauron has been with Melkor since the Years of the Trees. He and the Balrogs fulfill the most important roles in Melkor's regime. He had no reason to fear them because they were his “allies”.
So far I have learned that Galadriel is powerful enough to have a good chance. It is not clear whether she really would have to die or not. I personally think now that beating a Balrog could even be done without much physical power, e.g. by building a trap that will lead the Balrog to drown or fall very deep.
Balrog appears as one of the final bosses on Street Fighter X Mega Man. His weakness is the Mega Buster and the Soul Satellite.
He would not have been able to prevail against it otherwise. As far as I understand it, the Istari were specifically forbidden to use their full power in furtherance of their mission, which was to organize and inspire the Elves, Dwarves, and Men in their fight against Sauron.
Peter Jackson wanted to show a hideously deformed orc, one that would aptly convey the "ugliness" of Mordor. The upper left side of Gothmog's body is swollen and pock-marked from a disease of some sort, as described by the design department. His pale, yellow skin may also be a consequence of this illness.