Even in its Third Age, the world was yet full of places where time lay still and gathered mould and rust, forgotten by all that was. The glory days of Khazad-dûm rested in one of these places in the depths, where the only thing that moved seemed to be the dust that descended on everything. But it only seemed so; in the forgotten chasms moved fire and shadow - terror went before them - there, where Dwarves should never have dug, for the dark flame had long lived there alone and no longer wanted more.
And in the depths there was also a goblin's nest dug into the wall of the quarry, the only one of its kind in that place. It had been made by one single lonely goblin, who lived in it when he ventured down into the shafts on his expeditions. He had dug nothing else down there, nor ever brought the precious things he found up into the city; and on some days he simply lay in his nest and waited for the monster of the ancient world to pass by, hoping for a glimpse of the god of Moria.
For long before his birth, when goblins had first arrived at abandoned Khazad-dûm, the spirit of fire had shown itself to them and at once recognised in them the handiwork of his former master. "You cockroaches of the Dark Lord," was how goblin lore said it had greeted them. "I am Durin's Bane, spirit of might, in our lord's name I sow death and terror in war and myself die not; I am a maia, equal to your new lord, and among my kin have sung into existence this fading world."
The goblins had then fallen to their knees and taken as their god this lonely demon, who remembered the first days of their slavery as though it had happened yesterday, and feared it as much as they sought refuge in it. The goblin seeking the splendour of its flames also feared, but yearned for it more. And today his long wait was rewarded, for the first rays of the sun that rose in the dark lit the western wall of his nest and made the precious stones he had collected therein glow.
Had the goblin known the Elven legends of the creation of the Sun, he might have been able to compare the Balrog's course to Arien's journey across the sky. But beautiful and terrifying was this fire all the same, and he gasped for breath in the heat. As the Balrog walked past his home, it was momentarily bathed in that wonderful burning sensation. "I am..." the goblin panted, "a cockroach of the Dark Lord, I am but a mountain-goblin, and in war I'm among the first to die..."
Durin's Bane slowed down, but did not turn his head to his servant.
"I am Ghâshsag," said the goblin louder. For a moment the shadow and fire and smoke stopped his breath. "I took no part in the world's creation and I won't be there to destroy it, but I'm often lonely in its innards too, and I long for..."
Perhaps it was the goblin's attempt to sound formal, perhaps it was his preposterous insolence; the Balrog turned its head and bared its fangs in either grimace or grin, and then continued its journey. The silvery-shining veins of mithril that no one any longer mined and no one ever again would glittered now in the heights, golden with the fire's glow like burning stars in the sky under the mountain, and Ghâshsag couldn't resist descending from his nest and following the Balrog whose fire hurt but didn't kill.
Who knows what went through its mind in that moment? It could have destroyed one worthless rat effortlessly, but just as well could it simply not bother. Ghâshsag, for his part, followed it as its happy shadow as his own shadow grew in the Balrog's light, terror went before them; fire and darkness rained down from the sky over Moria, and neither burned him. Ghâshsag, a mere mountain-goblin, played with a god without being seared, and his heart was filled with joy.
I nicked the "terror went before them" bit right from the Balrog description in the Silmarillion.