I like the suspense before the appearance of the Balrog:
'We cannot leave you to hold the door alone!' said Aragorn.Makes you wonder and worry what nasty beastie could affect Gandalf that way.
'Do as I say!' said Gandalf fiercely. 'Swords are no more use here. Go!'
_____________Suddenly at the top of the stair there was a stab of white light. Then there was a dull rumble and a heavy thud. The drum-beats broke out wildly: doom-boom, doom-boom, and then stopped. Gandalf came flying down the steps and fell to the ground in the midst of the Company.
'What it was I cannot guess, but I have never felt such a challenge. The counter-spell was terrible. It nearly broke me. For an instant the door left my control and began to open! I had to speak a work of Command. That proved too great a strain. The door burst in pieces. Something dark as a cloud was blocking out all the light inside, and I was thrown backwards down the stairs. All the wall gave way, and the roof of the chamber as well, I think.'Finally, just before the Bridge:
It came to the edge of the fire and the light faded as if a cloud had bent over it. Then with a rush it leaped across the fissure. The flames roared up to greet it, and wreathed about it; and a black smoke swirled in the air. Its streaming mane kindled, and blazed behind it. In its right hand was a blade like a stabbing toungue of fire; in its left it held a whip of many thongs.