This came up somewhere else... is it actually written in LotR or perhaps the Simillarion that the Balrog in Moria was a physical creature and part of the Dark Lord's army (what Sauron was part of) and when he was beaten, it fled to the roots of Moria (which was my thinking)? Or was it a "metaphysical" evil that came out of the dwarves greed for mining so deep for precious metals like mithril?