This was the first incarnation of the Balrog brought out as a promo to preview the Mines of Moria, and it was included in the set as a common. This Balrog is designed as what people call a “splash minion” aka a minion that can go in any deck. Given the fact that in the days of Fellowship block everybody had to pass The Bridge of Khazad Dum he was included a lot, especially during the beginning. Eventually people started teching versus the balrog by playing more stealths, armor, and other cancelling effects, which resulted in the general opinion that the Balrog was doing too little, and his appearance dwindled near the end. And when the Bridge was rotated out he practically became useless.

If you do want to use the Balrog, consider using some support cards such as The Balrog's Sword and Whip of Many Thongs, as they can make a big difference to his effectiveness. It's possible to use this Balrog in The Two Towers at Caves of Aglarond, Cavern Entrance or Caverns of Isengard, although it might be more prudent to use Terror of Flame and Shadow, who does not have to be played at an underground site.

Archery decks. He skips the archery phase.

Site paths that dont go underground, cancellers.

Not really a deck specific minion, it could be found in all kind of decks.