Batman and the ponies got off the train and ran immediately into Canterlot specifically the castle that housed the two Royal Sisters. The sun was already up at this point, meaning that they were running out of time to prepare for the oncoming battles and changeling invasion to the city, "Open up!" ordered Batman, slamming his fists on the door. It creaked opened a few minutes later revealing Princess Celestia as she peeked.
"Oh. You must be Batman. Even though you're still a fugitive, I shall grant you permission to speak this once. Why have you and mus subjects come to me for?" the white alicorn asked. The Dark Knight silently responded by dropping the body of the changeling he interrogated on the ground next to the princess. The changeling they had captured was chained up and being escorted as well to the dungeon for further interrogation.
"We have to talk."
The Royal Guards escorted Batman, Spike and the ponies through the castle with Celestia leading them inside, "It is a shame we no longer have the Elements of Harmony to aid us... But this Ra's Al Ghul... You mentioned that he is indeed a powerful threat? How did he manage to ally himself with the changelings?" she asked. The heroes had explained everything to the Sun Princess up until the traumatic experience of travelling through eachother's nightmares with the aid of Princess Luna, culminating in exploring the vigilante's very own broken mind and learning more about him.
"The Elements of Harmony?"
Celestia turned to him, "Yes. They were six ancient artifacts that represented a different aspect of friendship and were wielded by the very six ponies you know. Applejack was the Element of Honesty, Fluttershy was Kindness, Rarity was Generousity, Pinkie Pie was Laughter, Rainbow Dash was Loyalty, and Twilight brought them all together with the Element of Magic. We had to sacrifice them to save the Tree of Harmony from being corrupted." she explained, turning to the mares and dragon standing next to the Caped Crusader with them.
"I see. Have you prepared your soldiers for something like this?" he asked.
"Yes. In fact I prepared them all to face you as did my sister, but I see you are no longer a threat." Celestia revealed.
"Hm. You are smarter than I anticipated." joked Batman.
"I am a ruler, Batman. I have to do what is best for my subjects." the Sun Princess responded in a stern tone. "I shall craft a dome over the kingdom as soon as I can. But first we should warn my sister of the attack and rally her troops as well. The more the better chance we have."
"Good idea, Princess." Twilight said.
Rarity smiled as did Spike, "Yes. Please hurry though. I am afraid we do not have much time according to Batman." she said, watching the princess walk into her sister's room. Luna stepped out a minute later, needing no explanation from her sister to leave her own domain at such an hour.
"Greetings Batman. I take it thou's not here for a friendly tour." Luna noticed.
"Not quite. A changeling told me that my enemy Ra's Al Ghul has allied with an enemy of your's, Queen Chrysalis and her army, and is coming to Canterlot today. We wanted to rally your Night Soldiers and band together an army of sorts in order to combat the supposed army." Batman elaborated, crossing his arms while leaning on a wall. "Do you think you can do that quick. By the end of the morning I predict is when they'll arrive and we're already running low on time as it is."
"Yes, Batman. Let me find my troops." said Luna.
"Why the hay is everypony suddenly listening to you!?" Dash questioned in disbelief. "Who made you the leader here?"
Batman frowned, "When I'm your only chance of survival, you should listen. As much as I hate to admit it, there is an entire army after us, and I cannot do this battle alone."
"Aw. Batman really is a big softie!" cheered Pinkie Pie.
Batman grunted, "Don't push me, Pinkie."
"Sorry!"
Spike put a claw on his chin, "Kinda makes you wonder though... What can we do? We struggled the last time they invaded! And now what you're saying is that they have an immortal assassin with them?! That's too crazy even for me. What chance do we have against odds like that?!" he asked in a panicked state. His friends looked at him, all of them trying to figure out the same thing. Silence overtook the once busy room as they pondered.
Finally, after what felt like an eternity, Batman spoke, "It doesn't matter how the odds are whether they're against us or we have an advantage. What matters is that we try to fight. You're not giving up and neither am I. We have an army at our side as well, you have your magic and abilities and I'll be the one to face Ra's Al Ghul."
"Yeah... Yer right... We have to at least fight. Ah'm not lettin' Equestria fall to the changelings today." Applejack declared.
Dash chuckled, "I never wanted to stop. After all, I'd never turn my back in my friends!"
"I'll stand by you all no matter what!" Rarity chanted.
"I-I can never forgive myself if something happened." Fluttershy said.
"For Freedom!' shouted Pinkie.
Twilight smiled, "And I'll make sure that we all stick together through it all!"
The six ponies and dragon placed a hoof and claw atop one another. Pinkie looked at the brooding hero, "Come on, Bats! You know you want to join this epic chant!" with a roll of eyes, Batman placed his hand on top of the ponies, "Woo!" the pink pony cheered they all raised them, and Batman went back to thinking on his own.
"We should check on the princesses." he said, changing the subject at hand, feeling as though their time is running out faster than they initially thought.
The eight stepped out of the castle to see Princess Celestia and Princess Luna looking over a bunch of guards wearing armor with the sun and moon on it. The Royal Guards had been assembled, and the Sun Princess was giving a speech, "...Together, we shall defend Equestria from those who wish to see it in ruins! You're all trained for this very purpose! Now, the changelings are on their way with Queen Chrysalis and a new enemy calling himself Ra's Al Ghul! All of you with the help of Princess Twilight, her friends, and the Batman, you will fight for glory!"
The guards chanted, ready to fight. A bright light over took the surrounding area and shadows covered the sky. Ra's Al Ghul and Queen Chrysalis stepped out, the heroes getting into a fighting stance. Above them, changelings began to descend onto the kingdom of Canterlot. The time had come, the final battle had begun.
"Ah, Detective..." Ra's began to Batman, "Welcome to the reckoning of Equestria. This is where you shall die and this world too shall be purged of its sins!"
"Over my dead body, Ra's." growled the Dark Knight.
The immortal assassin pulled out his sword, "Oh. I'll do just that."