In order to maintain an element of surprise, a full disclaimer can be found at the end of the story.
The path of destruction could be traced all the way from Rivendell, through the Gap of Rohan, passing by the still smouldering remains of Isengard, then down through the lands of Rohan and into Gondor itself. It travelled as fast as a galloping horse, never once pausing, even when faced with armies of Orcs, Uruk-hai or the races of man who had flocked to the Dark Lords resurgent banner. None who stood before it lived to tell the tail, nor did any who got close enough to see it with malice in their hearts. Even the unblinking eye of Barad-dûr was unable to see the true nature of the being that approached them, but it's course was clear when it crossed the great river at Osgiliath, shattering the army of Orcs that had been laying siege to the Gondorian garrison on the west bank.
Enraged beyond words, Sauron dispatched the great army he had intended to use to capture Minas Tirith, under the command of the Witch-King of Angmar, against the Fellowship.
The battle that followed was short and brutal, turning Dagorlad into a killing field the likes of which Middle Earth had not seen for an age: the broken remains of the armies of Mordor lay burning in their ranks, many having never even seen the enemy. Those that had were reduced to ash, scatted to the four winds. It was only now, close to his stronghold, that Sauron could finally see his enemy. It stood almost as high as the walls of Minas Tirith, with three dome like structures on its long and broad back. It's skin was the colour of tempered steel, but strong enough to turn back the sharpest blade. Strange and unknown weapons studded its hide, spitting forth death and destruction as it continued its unending march towards the Black Gate.
Two of the domes rotated to bring the long tubes that they housed into line with the Towers of Teeth, and twin bolts of lightning shattered the ancient fortress, a rain stone and burning wood showering down over the last few defenders. The dauntless juggernaut continued on, the iron and wood of the gate itself offering only a moments resistance before it was ploughed underfoot as the metallic creature moved on, headed directly for Mount Doom. Every last resource of the Dark Lord was flung against the strange foe, but it was protected from physical harm by a strong enchantment, and not even the darkest magic seemed able to turn it from its chosen course.
Finally, it was the steep and rugged slopes of Mount Doom itself that stopped the beast, and it was forced to remain in place as the other members of the Fellowship continued on foot. But still the creature protected them, unleashing its full might against all and any who tried to interfere with the Ring-bearer's mission. Then, when the Ring of power had been cast into the fires that had brought it forth, the leviathan carried the Fellowship back across the lands of Mordor, through the broken Black Gate and back to Minas Tirith.
As it passed by the ruins of Barad-dûr, the sound of a great horn could be heard emanating from the leviathan, and a deep Baritone voice called out "For the honour of the Dinochrome Brigade!"
Bolo's were created by Keith Laumer.
The Lord of the Rings and all associated characters were created by J. R. R. Tolkien.
I own nothing but a warped sence of humour and imagination.