"Okay...what have we learned today?" Twilight asked hopefully.
A silence passed for a few minutes, then finally, a low tone spoke back. "We've learned...that if another pony does something I don't like...I shouldn't respond by attacking them with dark magic."
Twilight beamed. "Excellent! You're making progress, your majesty!"
A nudge in her side caused her to look down, wherein she saw that it was Spike, who looked to her with more than a little incredulity. "Since he's not really a King anymore, doesn't that mean you don't have to call him that anymore?"
But Twilight raised a hoof, speaking up in that proper Canterlot way of hers. "Be that as it may, it's always good to observe titles for the sake of formality and politeness."
Spike folded his arms, shaking his head slightly. "I never saw him throwing politeness our way."
Twilight ignored him, looking ahead to her guest. The, as Spike rightly pointed out, former King Sombra, who looked as menacing and as evil as ever. But, let it never be said that Twilight Sparkle would ever judge a pony based on their appearances. And so, as she had with fallen enemies in the past, she smiled to him. "Now then, I believe we need to involve ourselves in a few exercises to make sure we're in the proper mindset of what makes a good friend."
Sombra just stared at her, and in a way not unlike that of a predator stalking their prey. "What mindset would this be?"
Twilight clapped her hooves together. "We need to perform the trust exercise!"
Spike gave himself a facepalm. "Of course we do."
Then, much to the little dragon's unnerved surprise, he opened his eyes to suddenly find Sombra right up to his face, grinning wickedly. "I am more than willing to engage in an exercise of trust. So tell me, little dragon..." He reached forwards, placing his hoof upon Spike's shoulder. "What dark and terrible secrets do you wish to divulge to me? I promise I won't use them against you later," he said in the least-convincing way possible.
Spike's face was one of utter terror, and after glancing up to Twilight he watched as the mare, rather confusingly, giggled to the situation. "No, Sombra. Spike's going to fall backwards, and then you're going to catch him, proving he can trust you."
Sombra tapped his chin. "Hmmm...get them to trust me so they'll be less inclined to suspect me later. Yes, I can see the benefits."
Spike looked to Twilight, then back to Sombra, before settling on Twilight. "You...you did hear him just now, right?"
But Twilight waved him off. "Oh, I wouldn't read too deeply into it. Now then..." She clapped her hooves again. "Assume the proper positions please."
Spike sighed, walking over to Sombra and turning so that his back was to the latter. "This is so gonna end up badly, I just know it."
Twilight gave a firm nod as she looked to them both. "Alright...now fall backwards, Spike!"
Though it was clear to all that Spike had no desire to do so, he took a deep breath and did as he was instructed. Now, he'd fully excepted to just collide with the floor at that point, perhaps out of some twisted joke on Sombra's part. Instead, he surprisingly found himself falling into open hooves, supported so as to not fall. Glancing up, Spike looked on at the near-emotionless face of Sombra, before gulping loudly. "Um...thanks?"
Sombra gave a single nod, but he didn't say anything. By contrast, Twilight, who looked utterly giddy with what had just happened, spoke as though she were absolutely ecstatic. "Eeeeeeeeeee! I knew my teachings would work! This is remarkable! A vital first step on the road to recovery and redemption!" As Spike stood upright again, Twilight rushed over to Sombra's side. "With this, you've proved that you're capable of being trustworthy! It might not look like much, but I assure you, this is a very good thing."
Sombra, after staring at her for a time, shrugged. "If you say so."
Then, Twilight took on a look that Spike, in all his years of knowing her, signalled the arrival of an idea. "Oh! I've got it! Now that we've taken this first step, let's go further!" She charged over to her front door, opening it up and gesturing to the outside, to Ponyville. "Sombra, I'd like you to go out into Ponyville and try to interact with other ponies. You don't need to make friends right now, just talk to them."
Spike frowned, walking over to her. "Er, you sure it's okay to let him off the leash like that?"
But Twilight did not share his worries, as evidenced by the pat on the head she gave him. "Don't fret, Spike. I'm confident this will be a success." She turned to face Sombra, who looked from her to the door and then back again. Eventually, he nodded, walking over to the door and finally exiting. It was certainly an odd sight, seeing the ancient dark-magic-obsessed monarch walk among the ponies of Ponyville like everypony else, but it gave Twilight immense delight to see that he was taking strides to be part of the community, even if it was only in this small way. Slowly, Twilight closed the door, sighing contentedly. "Ah! This is a good day! I predict he'll be on the way to recovery in no time flat!"
Spike rolled his eyes. "Sometimes I think your optimism gives even Pinkie's a run for her money."
Twilight looked to him, slightly dismayed at his cynicism. "Oh Spike, you need to have greater faith in ponies. For as the great philosopher Haycartes once said..." Alas, she couldn't finish, as she wast taken aback by the sudden sound of screaming from outside, prompting her and Spike to rush out of the door. Sombra was out there, and as his earlier instructions had said, he was certainly not attacking ponies with dark magic. Instead, he was using said magic to mind control them into obedient servants, eliciting screams from everypony else.
After noticing them, he turned, smiling as if expecting a reward. "He did something I didn't like...so I made him stop. Is this right?"
Instantly, Twilight teleported to his side, along with a rolled-up newspaper, which she started whacking him over the head with. "No! Bad Sombra! No enslavement!"
Folding his arms, Spike chuckled. "I'd watch this any day...if it weren't so terrifying."
Author's Note
Who gets reformed and who doesn't is sort of a coin-flip in this show, and while I know I sort of did a reformation story for Sombra a while back, I figured it'd be interesting to write a little snippet where he went in the total opposite direction 
