Balrogs are race of Maiar/humanoid spiritual beings of the supernatural which have been corrupted into evil. They are tall and menacing, with the ability shroud themselves in shadow and flame. They can come armed with fiery whips, swords, and other weapons at their disposal. Only dragons have rivaled them in ferocity and destruction, at least for a time.
Since the days of the Ainur (God-like beings) the Balrogs have pleged their service to Melkor/Morgoth (The evil Valar/God), since then they have been reveered as his some of his most loyal and dreadful followers.