Uncertainty gnawed at Sombra like a parasite as he and his comrades entered the main cavern. At first glance it was shrouded - and he felt a twinge of fear. When they emerged from the cave and came face-to-face with his mirror self, he wasn't sure if even their combined might would be enough. From what Celestia had described (with Rarity and surprisingly Fluttershy had as well) - his mirror-self was a handsome, rather dashing stallion with gentle green eyes and pale grey coat, dressed in a magnificent ermine cloak.
Who, or rather what they found waiting at the top of a raised dais in the center of the cave was none of those things. Simply put, he had truly become a monster in body as well as mind.
Mirror Sombra's body had been warped by the evil within him; what appeared to be cracks in his coat with black substance oozing from them. His eyes now were bright red and had shrunken pupils. Even his mane had changed somehow - it was now spiked and stuck out in all directions and had a flowing motion, not unlike the royal sisters, though it unnerved the former king with how unnaturally it swayed. About his broad shoulders hung that same cloak, but now it was tattered and ripped at the edges - and traces of dirt and filth could be seen on the lining.
When he parted his fanged mouth and spoke, it was in a voice that was as cold and echoed like some bottomless cavern.
"Welcome... I have been waiting."
For a long moment, nopony spoke or breathed. It was as if a terrible trance had come over them. Even Celestia and Luna paused - awed and horrified at the sight. Through the gloom, the true Sombra looked out and beheld the mirror - just beyond his alternate self, exactly as Amore had described; tall and cracked, bearing the ancient, magical markings of Starswirl the Bearded.
"Is there not one among you with courage to speak? Are you all so afraid of me?" The mirror-being said with a cruel smile, eyeing them all one by one. When his gaze settled on the Sun Princess, for the briefest of moments, it softened. "Even you... my dear sweet Tia?"
Celestia opened her mouth as if to speak, but said nothing. She looked beside her at her sister, Luna who as was still as stone. There was a heavy silence.
It was Amore who broke in suddenly. "Come now, are we all to be cowed before this tyrant by a mere glimpse of him?" she growled. "We all know of what he plans to do, what he intends for us." She locked eyes with the mirror-being and stomped her hoof. "I give you the same counsel that I gave our Sombra many years ago: Do not journey down this dark path - for it is not too late for you."
All present were stunned by the Princess' words and looked up again. Mirror Sombra stared and for a moment he hesitated - and nopony spoke - and then his face twisted with a sneer.
"Repent of my ways?" He mocked. "You are a fool to think that the betrayer you call friend and I are the same. True, we share a name and a history - but we are different, he and I in all the ways that matter. He betrayed the Umbrum - just as he betrayed you, Princess Amore. He has you all fooled - but I know the truth - he serves nopony but himself. He loves nopony but himself."
"You are right." The true Sombra said, to the surprise of all. "We are different. You mistake my atonement for weakness and my love for my family as false. But lest you forget, we are also very much the same in many ways. We both have loved and lost, but where you sacrificed yourself nobly - for a cause you believed in, I sacrificed the mare I loved out of fear and revenge. A grievous error on my part, one which I lived with every day for a thousand years. A mistake I have worked all days since to try and atone for."
"And it is one he has atoned for." Said Twilight Sparkle. "Like we told him - he has reformed. He saved himself from a terrible fate. You can too!" She looked, pleadingly at the mirror-being. "You and I were friends once, for a little while. You asked me to give Princess Celestia your regards. She is here now."
All eyes turned to Celestia, who stood to her full height - a determined look on her face.
"Indeed. If there is anything left of the stallion I loved... no, still love within you - please, come back."
"I am afraid it is too late for me, Tia." The mirror-being said very softly. "But it is not too late for him." He pointed with his hoof towards the true Sombra. "Now, all will be made right again."
"What do you mean?" Sombra demanded, somehow meeting his mirror self stare-for-stare. "Speak sense!"
"For centuries, I resisted the dark powers - and how did cruel fate reward me? By mercilessly stealing away the mare I loved most, when all I wanted was to be with her - always." The mirror being slowly trotted forward - each step in even strides. "But you... you accepted the Umbrum without question - and gave in to the darkness. Do you not see the unfairness of it all? You did not deserve redemption! You still do not!"
"You are wrong!" Said Celestia. "Sombra has earned his redemption. So can you. Relinquish the dark power within you and let us destroy it - together." She gestured to her fellow Alicorns. "With the five of us, there is no limit to what we can accomplish."
Mirror-Sombra shook his head slowly. "No, Tia. The dark power cannot be destroyed. Only contained. It needs a living host - and so it will seek out and possess any nearby. Even if you draw it out of me, it will simply find another. Yet, there is a solution."
Sombra stared at his mirror counterpart, already aware of the proposition. "You want me to take back the dark powers then? To become a monster like you once again? I refuse."
Mirror-Sombra chuckled. "Of course you would. But that is not what I meant. The solution I speak of is for all things to become part of me."
Sombra was disgusted at the thought. "You wish to merge all existence to the dark power?"
"Indeed. The merging of all worlds is just the beginning. If I can harness this power - I can use it to reshape all that exists as per my vision. Imagine: A world free of suffering. Free of the futile struggle between good and evil. Free of death, even!" He stared down at Celestia with a longing look. "And you, Tia - can rule by my side."
"I would rather die."
"Hmn. Pity." The mirror-being seemed unperturbed. "No matter. Now is the time you all stopped resisting and joined me in remaking this world!"
"Hear me, shadow-pony." Celestia said slowly, stepping forward and pawed at the ground with her hoof. Every word was laced with venom. "I speak to you now not as the Sombra I knew and loved, but as the monster you have become. In memory of the Unicorn you once were, we will defeat you again!"
The mirror-being laughed; an ugly sound. "Foolish princess." He boomed. "You forget. I am still the same being as your friend there. Any harm you inflict upon me will be affect him as well. Kill me, and you kill him."
"Who said anything about killing you?" Twilight said, gesturing to her friends who reared in response. "We have a trick of our own."
Mirror-Sombra laughed again. "I know all about your silly 'Rainbow Power', Princess Twilight. I am not so easily defeated as that brute Tirek, because unlike him - I know you - and I know your weakness. How can you and your friends hope to defeat me, when you cannot even defeat yourselves?"
Before anypony could ask what he meant, there was a flash of light - and when it subsided - before them stood not one foe, but seven.
Twilight found herself staring at a mirror-image of herself, only without her Alicorn wings - a contemptuous scowl on, well, her face. Beside her, the other Element-bearers were doing the same - only there were subtle changes in each of their counterparts. Unlike her friends though - all of them had the same sneer.
"I think you'll agree that this is a fair fight." The mirror-being went on, his grin widening. "Now if you don't mind, my dear, sweet Celestia and I have some catching up to do!"
Before anypony could blink - he reared back and fired a magical blast from his horn. In another flash, the element-bearers disappeared, leaving only a stunned Sombra and the princesses behind.