Before they reach the bridge, they are surrounded in the great hall of Dwarrowdelf by an entire horde of orcs.
But behind them they hear a roar and see a glowing light. The orcs become frightened and scatter.
Gandalf says that the creature approaching them is a Balrog, an ancient demon that is too great for them.
They continue running for the bridge and, after some close calls with crumbling stairways and orc arrows, they make it to the bridge of Khazad-dum with the Balrog hot (very hot) in pursuit.
Gandalf is the last to cross the bridge and pauses at its center to confront the Balrog. They clash in a struggle of magic and fire until the bridge crumbles under the Balrog's weight.
But as it falls, its whip catches Gandalf and pulls him down into the depths with him.