The Balrogs were Maiar corrupted by Morgoth during the creation of Arda, who cloaked themselves in shadow and flame and carried whips and swords. Famed Balrogs include Gothmog, slain by Ecthelion, and Durin's Bane, slain by Gandalf.
Gandalf was able to kill the Balrog, but he never considered fighting Smaug. Instead, they sent Thorin and his company of dwarves to do the job while he dealt with the Necromancer in southern Mirkwood.
In addition to the twenty-eight named above there were also Gothmog, slain by Ecthelion, and another Balrog slain by Glorfindel... coming to a total of 30 plus however many slain by the men of Rog, however many slain during the fall of Thangorodrim, and the Moria Balrog and any other survivors (35 minimum).
Glorfindel was one of the mightiest Elves of Middle-earth. During the First Age, he was the lord of the House of the Golden Flower of Gondolin, and died fighting a Balrog.
The Elf Ecthelion leads the charge against the Orcs, and fights Gothmog, the greatest Balrog; they wound each other and both fall into the king's fountain in Gondolin; both drown.
Secondly, Balrogs were the epitome of malice and power while Sauron only ever won one of three fights he ever had. Sauron was the great deceiver. His power lies in planning, subterfuge, manipulation, and corruption.
As the legendarium became more formidable and internally consistent, and the Balrogs more terrible, this number was much reduced. In the end Tolkien stated that there were probably "at most" seven Balrogs: In the margin my father wrote: 'There should not be supposed more than say 3 or at most 7 ever existed. '
The Battle of the Peak was Gandalf's name for his climactic battle with the Balrog known as Durin's Bane, on the peak of the mountain called Silvertine. Gandalf pursued the Balrog from the deepests dungeons of Moria to Durin's Tower, climbing through the whole Endless Stair.
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.
Gothmog, one of the first Balrogs to become Morgoth's allies in the First Age, is regarded as the most powerful Balrog in Middle-earth. As the High Captain of Angband, he is given the title 'Lord of Balrogs'. Although he carries a fiery whip to battle just like the rest, he is also renowned for having a large black ax.
The Balrogs, or Valaraukar (singular Valarauko, Quenya for "Demon of power"), were three to seven Maiar seduced and corrupted by Morgoth into his service, turning into dreadful Úmaiar and scourges of fire cloaked in shadow.
Although both the Balrog and Smaug are creatures of fire, and Gandalf is able to match them thanks to the power of his fire ring, defeating the Balrog takes far more spiritual prowess, which is why Gandalf is able to essentially 'level up' and comes back as Gandalf the White, whereas defeating such an immense physical ...
Still, the dragons proved more effective in battle. Then there's Gandalf the Grey, who managed to kill a Balrog single-handed, but decided to manipulate some Dwarves and one very reluctant hobbit into dealing with Smaug.
Ecthelion stood no chance against the Lord of Balrogs, and lost his sword in the brief struggle. But then Ecthelion leaped forward, and stabbed Gothmog in the breast with the spike atop his helm. They both fell into the Fountain of the King, where Gothmog, if not already killed by the spike, drowned with his opponent.
The company fled with Gandalf, but the Orcs and the Balrog, taking a different route, caught up with them at the Bridge of Khazad-dûm. At the Bridge, Legolas instantly recognized it as a Balrog, and Gimli recognized it as Durin's Bane.
Glorfindel fought the Balrog and succeeded in slaying the monster, but was himself mortally wounded. His body was recovered by the great eagle Thorondor and buried under a stone cairn, where afterward grew yellow flowers.
It's worth noting that Sauron wasn't a Balrog. His appearance was dark and wreathed in fire, but he wasn't physically the same as a Balrog. On a surface level, it's a bit of a plot hole. After all, Sauron and the Balrogs were both Maiar, and they both served Morgoth's evils during the First Age.
By my understanding, the Balrogs were seen as undefeatable up until the battle of Gondolin, when up to eight were slain (3 (?) by the House of the Hammer, 4 by Ecthelion & 1 by Glorfindel), depending on the account read. Gandalf obviously also kills Durin's bane in Moria during LOTR, bringing the total up to 9 (?).
6 Gandalf. As previously stated, Gandalf is a Maia and is therefore superior to Elves regarding power. Still, this isn't to say that an elf couldn't develop enough skill and power to defeat a Maia. As a bearer of one of the Three Elven Rings (Nenya), Galadriel would have an edge against Maiar like Saruman and Radagast.
Sauron does not fear Durin's Bane because essentially they had been allies back “in the olden days". Both Balrogs (all balrogs) and Sauron chose to serve Melkor/Morgoth.
If the Balrogs are corrupted Maiar, then would Saruman eventually become one? Saruman wouldn't become a Balrog, because his nature is inherently different and he's kind of stuck in the body he's in.
