Balrog of Moria

This mighty creature was perhaps the last of his kind. Certainly no Balrog had been seen in Arda since the Great Battle that ended the First Age, and only the Elves and the Wise (including the Istari) knew of them beyond the oldest tales and dimmest rumors. The Dwarves that reentered Moria near the end of the Third Age had dug deep, perhaps more deeply than had any mortal creature before, and uncovered this menace from its self-induced coma in hiding in the roots of Moria. Aroused and rested, the Balrog once again felt its power and pride; perhaps, too, it felt the Power its Lord's lieutenant, Sauron of Mordor, reaching into the world. It killed two of the Dwarf-kings and gathered to itself a mighty army of Orcs and Trolls from the surrounding area by the force of its will alone. With these troops it drove the surviving Dwarves from Moria and established itself as Master.

What it would have done if it had captured the One Ring? It is doubtful that it would have surrendered it to Sauron. Would it not rather have wielded it itself, confident that it was a better pupil of the Foe of the World than the Lord of Mordor? The answers are unknown, since Gandalf the Grey, an Istar and thus a Maia himself, fought and vanquished this awful Lord of Moria — sacrificing his Fana in the process.