The Bridge of Khazad-dûm is the fifth chapter of Book Two of The Fellowship of the Ring.
Having discovered Balin's Tomb, the Fellowship finds the Book of Mazarbul and learns of the last days of Balin's attempt to recolonize Moria. Soon afterwards, they are attacked by orcs and a cave-troll, which after some difficulty are defeated. Gandalf senses the presence of Durin's Bane, and the Fellowship flees to the Bridge of Khazad-dûm. The Balrog pursues them, and at the bridge, Gandalf confronts the creature to allow the rest of the company to escape. The wizard is able to cast the Balrog into the depths below the bridge, but is dragged off himself and lost. The rest of the Fellowship, believing him to be dead, flees and escapes Moria by the east-gate.