You cannot pass

"I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass. The dark fire will not avail you, flame of Udûn. Go back to the Shadow! You cannot pass."
—Gandalf to the Balrog.
On January 15, 3019, the Fellowship came to the Bridge of Khazad-dûm pursued by Orcs and a Balrog of Morgoth. Gandalf stopped on the Bridge, standing in the middle of the span, allowing the others to escape. He leaned on the staff in his left hand and held the sword Glamdring, gleaming cold and white, in his right. The Balrog stepped onto the Bridge, facing Gandalf, and the shadow about it reached out like two vast wings. It raised its whip, and the thongs whined and cracked. Fire came from its nostrils. But Gandalf stood firm and declared that the Balrog could not pass. They fought, and the Balrog's sword was destroyed. Then the Balrog leaped full upon the Bridge, and Gandalf lifted his staff and smote it upon the bridge. His staff shattered, but the Bridge cracked at the Balrog's feet. The stone beneath the Balrog broke and fell, taking the Balrog with it into the abyss, but the thongs of its whip snared Gandalf about his knees, and Wizard and Balrog plummeted together into the depths of the mountain. Gandalf cried, "Fly, you fools!" and was gone