Well, the first course of action (at least in Twilight's mind) was to free the other alicorn princesses from their crystalized states. It took very little effort to get the staff to do just that. And when the other princesses were freed, it took them a moment to get their bearings and realize where they were.
Everypony was very glad to see the princesses again. And so was Spike. As for Grubber, he stayed a ways away, sensing that the princesses might not be pleased to see him so soon after being freed. He felt it better to hang back, stay by the still frozen Tempest, until she was also set free. Afterwards, Tempest took several deep breaths as the shock of her sudden decision slowly wore off. As it did, she remained where she stood. She didn't move an inch. It never even occurred to her that it might be best to run away again, like she had done before.
Instead, the hornless unicorn took in the sight of the ponies and Spike embracing the other princesses, exchanging pleasantries and openly showing their affection towards them.
Princess Celestia was the first of the trio of recently liberated princesses to find the words she wanted to say. "Thank you, all of you. It would seem Equestria yet again owes you all a great debt of thanks," Then her eyes slowly took in her surroundings, particularly the missing roof. "Goodness, Twilight!" She exclaimed with disbelief! "What has happened to your castle?!"
Twilight sheepishly replied. "It's a long story, Princess Celestia. But at least I can safely say that Equestria has seen the last of the Storm King. And so too have all the other kingdoms he's ever conquered or laid eyes on."
Princess Luna spoke up next, asking the question. "So, did you all journey beyond the borders of Equestria in my place? Were you able to find this... Queen Novo?" She shot off a most puzzled look.
Spike nodded. "We did," Then he looked to Twilight. "Although, things got kind of... complicated. Let's just say a certain somepony is probably going to owe Queen Novo a very big apology."
At that, Princess Cadence took notice of the guilty look on Twilight's face and saw fit to ask her. "Twilight, what did you do?"
Twilight gulped and swallowed hard. "Something I'm probably going to regret for the rest of my life. I wouldn't be surprised if Queen Novo never wants to speak to me or see me ever again. And I wouldn't blame her. What I did in her kingdom..." She looked down at the ground in shame. "It wasn't my finest moment."
"Oh, you mean like the time you enchanted your old doll and made all of Ponyville fight over it?" Princess Celestia half teased, half inquired.
The youngest of the alicorn princesses sighed. "Yeah, something like that. I mean, I didn't use any enchaments if that's what you're thinking of. But I still did something that... now that I think about it more... it really doesn't seem like I should've ever thought it was a good idea," She gave off a nervous laugh. "And I'm not the only one who has a lot to answer for. Uh... in fact... that's kind of what I was hoping to talk to you about."
But there was no need for the other princesses to ask what Twilight meant, for they could all see Tempest and Grubber standing off in the distance.
Grubber tried to put on an innocent smile as he thought for a bit about what to say. Unable to come up with a good response, all he could do was force out a quick "Uh... um... hello?"
Princess Cadence was the first to interrogate Tempest, seeing as she had been the first to be taken prisoner by the hornless unicorn. "What are you doing here?" She demanded in a noticeably stern and rightfully angry tone of voice. "Are you here to surrender and beg for mercy?"
Tempest bowed her head as she struggled to maintain her composure. "Yes, I am here to surrender. But not because I wish to beg for mercy or forgiveness. I can't ask any of you to forgive me. I know you don't trust me or have any reason to believe a word I said, and I know I have no defense for anything I said or did to you."
"Took the words right out of my mouth," Cadence remarked even as her tone of voice did not rise in the slightest. "What did you do to my husband and my daughter? If you hurt them the way you hurt me..."
Tempest quickly answered back. "No no no! I assure you, they're unharmed. They'll be set free with the other prisoners very soon."
"Uh, in fact, why don't we get on that and get out of your... manes, is it?" Grubber sheepishly proposed and immediately tried to make a run for the castle doors.
Princess Luna refused to let the little hedgehog move an inch from her sight. She instead used her magic to pull him close. "You don't really think you're going to get away that easily, do you? You and Tempest showed up here on behalf of an invader, you attacked our subjects, and now I hear you've taken many of them prisoner. And some of them were innocent children who never did anything to you."
Princess Celestia nodded quite sternly. "I couldn't agree more with you, Luna," She turned her attention to Tempest and Grubber. "After what you have said and done, you're going to have face the consequneces of your actions. You will have to be punished. The only question now is: How difficult do you wanna make it for yourselves when you inevitably stand trial?"
Twilight immediately jumped in, seeing fit to protest. "Princess Celestia, Princess Luna, Princess Cadence, you must understand that Tempest is at least partially responsible for the Storm King's demise. She sacrificied herself to save my friends and I from him, and in doing so caused him to turn to crystal and then shatter into pieces. If it wasn't for her help, the Storm King would've won. He even double crossed her, made her believe he could fix her even though he had no intention of doing so."
"Even so, she helped the Storm King to become so powerful in the first place," Celestia pointed out. "Like it or not, she and Grubber cannot escape without punishment."
Twilight then argued. "There's a time for that later. For right now, I think everypony and everyone just wants to put this whole invasion behind us," She proceeded to plead. "Please! Let the Festival of Friendship resume! When it's over, then we can talk about Tempest and about what a fitting punishment for her should be. Give everyone a chance to heal before we rush into judgement. If there's anything this whole ordeal has taught me, it's that if we rush into things too quickly and don't listen to others, we often end up making things worse."
The other alicorns were silent for a little bit, apparently deep in contemplation about what Twilight had proposed. All the while, Tempest and Grubber waited with anxious and uneasy expressions. They knew their fates were out of their control, and that even Princess Twilight held very little say in what might happen to them. They couldn't think of anything to say or do that would convince the skeptical princesses that they were indeed sorry for what they had done.
At last, it was Princess Cadence who seemed to sum up the feelings of the princesses who had previously been trapped in crystal. "You make a very good point, Twilight. You are the princess of friendship, after all. And you're right, it wouldn't be wise to rush to judgement now. We need time to gather all the facts and weigh our options."
Princess Luna then added. "Besides, I have a feeling that what most of our subjects need now is a chance to unwind, to process what they've been through. The Festival of Friendship could go a long way towards helping Equestria and its allies heal."
"Besides," Princess Celestia chimed in. "There is still the matter of the concert. It would be a shame for Songbird Serenade to come all this way for nothing."
Twilight beamed a happily smile and rushed up to the other princesses, hugging them all tightly. "Thank you, all of you!" She then turned back to her friends. "Well, what are we waiting for?! Let's get the Festival of Friendship back underway! I know that with the help of our new friends, we can still salvage it!"
All of Twilight's friends cheered, and Pinkie Pie cheered the loudest of all! "Now's that the Twilight we all know and love!" Just like that, the Festival of Friendship was saved!
Sometime later, all the cages had been opened and all the Festival of Friendship attendees had been set free. All of them were extremely grateful to be out and about again.
Twilight took the liberty of apologizing to one of the last ponies to be set free: Songbird Serenade. "I'm really sorry about all of this, Songbird. I'll understand if you have second thoughts about performing after all of this."
Songbird Serenade brushed off the concern. "Hey, it's all a part of being a celebrity. You just gotta roll with the punches. As they say: The show must go on!" Once set free from her cage, she needed very little time to compose herself and prepare for the concert.
Once the stage was cleared and all the damage from the short lived invasion was cleared up (thanks in part to the magical powers of the staff, though using it to repair said damages ended up draining most of the magic within it. It was still very fortunate that once it was destroyed, all the magic returned to its original owners), the crowds gathered around the stage as they waited for Songbird Serenade to begin her performance.
Before the concert began proper, Twilight came trotting out onto the stage with a microphone headset attached to her left ear. She spoke to the audience, motioning for the applause to die down. "Thank you one and all for sticking around in spite of the unfortunate setback we endured," She turned back to the stage and gestured. "Before we get to the moment I know you've all been waiting for, I'd like to take a minute to acknowledge the help of some new friends who made this victory over the Storm King possible."
A moment later, out onto the stage strolled Capper and the pirates. Capper now wore a long, flowing red cape and a matching top hat, both of which had been sewn for him by Rarity. The pirates remained dressed in their usual attire, and alongside Capper they bowed to the audience, basking in their thunderous applause.
Princess Skystar swooped out onto the stage soon afterward. She wasn't wearing anything special, and she could barely contain her excitement. Yet all the while, her thoughts turned back to her mother, who was sure to disapprove of such a public display on top of the defiance of her orders. The teenaged hipogriff princess was already dreading the conversation that was sure to await her back in Seaquestria. So she made sure to enjoy every minute and every second of freedom she still had.
At last, Capper, the pirates, and Princess Skystar all left the stage and the applause died down. Twilight then gave a nod as she flew off the stage.
Spike stepped out from behind the curtain, a large microphone held within his claws. "Alright, mares and gentlestallions, colts and fillies of all ages! It's the moment you've all been waiting for!" He declared in a thundering tone of voice. "So please, put your hooves, claws, or whatever you got together, and give a warm welcome to Songbird Serenade!"
Songbird Serenade quickly swooped down onto the stage from above, spotlights shining brightly on her! The sun had set across Ponyville, and the moon was shining brightly! Lights around the edge of the stage illuminated everything for miles around! And the crowd's applause was borderline deafening! Yet it seemed not to bother her at all as she just cleared her throat, the stage's speakers carrying her sweet, gentle voice far and wide. "Hello, everypony and everyone!" She greeted with a wave of her hoof! "There's been a slight change of plans. I'm debuting a brand new song right here, right now, courtesy of a suggestion from Princess Twilight Sparkle! And I'm told it is to be dedicated to a very special pony who has helped to make this special occasion possible."
A shocked hushed fell over the crowd as music slowly began to play.
Songbird lifted her head and raised her voice as she began to sing. And as she sang, her cutie mark began to glow brightly.
Suddenly, in the middle of the song, fireworks shot up into the sky from behind the stage! The audience oohed and aahed, assuming that the fireworks were a planned part of the concert.
But they weren't. The fireworks were courtesy of Tempest, who had found enough energy within that stub on her forehead to shoot off beams of energy. And for whatever reason, they had been transformed into magnificient fireworks.
Twilight and her friends were all amazed by this display from the hornless unicorn! Twilight especially. "Tempest! I never knew you were capable of something like this!"
Tempest slowly nodded. "It's all thanks to you, princess. You helped me to find my way back to the light. You believed in me even when I had given up on myself. If not for you," She shuddered and briefly shut her eyes. "I can't imagine I would have anything to be smiling about right now," She opened them back up, smoberly stating. "Whatever may come next, at least I hope this counts as a small down payment for my atonement."
Just then, a loud screeching noise split the air! Everyone looked up, and beheld a most magnificient sight! A large bird like creature with a majestic white coat and with a golden crown atop her head came flying in from seemingly out of nowhere. Accompaying the creature were several smaller bird creatures that all appeared to be wearing helmets and carrying spears.
Princess Skystar realized at once who it was. "Mother!" She cheerfully exclaimed! Momentarily forgetting about all she had done, she rushed over and embraced her female parent in a tight hug!
Queen Novo smiled. She was at the very least glad to see her daughter was safe and sound. "Princess Skystar," She appeared to chuckle. "Thought I'd find you here."
The hippogriff princess blushed and sheepishly began to say. "L-listen, I can explain everything."
Queen Novo just grinned. "Oh I'm sure you can, sweetie. And I'm proud of you for standing up for yourself and standing up to the Storm King. Bravery is the hallmark of a good leader," The grin slowly faded. "But you still disobeyed me and went behind my back. And for that, you are so grounded! You won't be leaving the kingdom anytime soon!"
Skystar gulped and swallowed hard, accepting the punishment that had been inflicted upon her.
Princess Celestia came trotting upon the scene a moment later, quite surprised by the queen's unannounced appearance. "Queen Novo!" She happily exclaimed! "It's been so long! You're looking well."
Queen Novo nodded back. "Same to you, Princess Celestia. Sorry for not keeping in touch like we promised. The past few years have been... difficult."
Celestia slowly nodded in reply. "I'm sure they have been. And I regret that I didn't try to find out sooner about what had happened to you."
The queen was quick to reassure the sun princess. "You couldn't have known," Then she cleared her throat. "But I didn't come here just to catch up, or just to bring my daughter back," Eyeing Twilight and Tempest, she sternly and firmly declared. "Let's just say that some certain someponies have a lot to answer for!"
Both Twilight and Tempest gulped in unison. They knew full well what the hippogriff/seapony queen was talking about. And it was anyone's guess as to whether she would be willing to accept any sort of apologies from them.