"Lulu!" I cried happily, wrapping the goddess in a hug. She was taller than me, but I'd gotten used to it. I could feel her tears gently rolling down my neck. "I didn't think I'd see you again!"
"I didn't know you were down here!" she replied. "But I don't know HOW I'm down here, plus these caverns became haunted after the train crash."
"Well, that explains the things in the hidden passage." I replied, unlocking the embrace. "Anyway, two things."
"One?" she asked.
"I know the way out, but it'll require flying. You do the flying, I can guide, because I can't fly for obvious reasons." I said.
"Well, that's alright with me, as long as we get out of these caves." Lulu nodded. "And two?"
"Nightmare Moon has been hanging onto my concious mind and living alongside me." Luna reeled back slightly. "Why're you worried? She's been helping me along the way, apparently, and if she had her own body would be willing to 'experiment' with you, if you know what I mean."
"I'm worried, Thunder, because she corrupts minds if she's in them for a long time. She's been in yours for a year, slightly less. I mean, yeah, it'd be good to know what a mare's ass is like, or to feel a mare licking my ass, but if I gave her a body for her own purposes, she'd use it for evil. So, bad and good, then."
"Hi, Luna."
"What?!" Luna panicked, frantically looking around. "Who's there?!"
"Me." There was an oddly entrancing feeling as a dark mist poured from my mouth, nostrils, and eyes, before it passed. Stood before the two of us, in the shape of her original self but as smoke, was Nightmare Moon. Her eyes seemed to be normal, except glazy. "Good afternoon, Luna." My marefriend looked terrified, cowering slightly at the more imposing, had sharper teeth, and more evil looking eyes. (And dare I say the most succulent flanks I'd seen for a while. Dayum, I'd eat them cupcakes all night lo-TANGENT, SORRY, THEY'RE JUST REALLY NICE FLANKS.)
"Erm...aftern-noon?" she whimpered. Rolling her eyes, Nightmare reached out gently, so as not to scare Luna, and put a hoof to her cheek with a slight smile. It was so touching, my heart melted. ~~But I don't have a heart. I have a Batheart. Because I'm Batman.~~
"Listen, I'm sorry." Nightmare whispered, moving Luna's head to look at her. "You encouraged foals to face their fears. Now I'm facing mine: The prospect of remaining unloved forever, just a foal's tale. I'm more than that, because we both know I'm Batman that I'm no tale. I'm so real, in fact, that I've pledged allegiance to you two. If you cannot beat thy enemy, join their ranks." Promptly, Nightmare bowed down to Luna, who still looked bloody terrified. "I'm yours to command, Princess."
"Well, first command should be to get a more normal voice." I cut in. "I mean, I know you've remained silent for a whole year, and now you're pretty much a cloud of sentient steam, but the voice will really attract attention. If I might make a request, use the most memorable voice you can."
"Your wish is my command." replied Nightmare. She cleared her throat, concentrated, and spoke again. "How about now?"
"You sound like Duke Nukem."
"Is that good?"
"Nope. Try...I don't know..."
"Use my voice, but in a really sultry tone." Luna giggled. "Pleeeeeease?"
"Very well." replied Nightmare Nukem. Once again, she cleared her throat. "How about now?"
Poomph.
Poomph.
"I sound that hot, huh?" Luna asked, blushing slightly.
"Sounds even better when your ass is in my face," I laughed, "Anyway, we can do this later. Just use your original voice, Nightmare, until we can get you back to the chambers."
"Ugh, very well." She cleared her throat in an incredibly arousing way, before nodding. "Let's get to escaping these caverns. Thunder, I shall be riding in your head." Before I could say anything, the mare disappeared, before pouring into my mouth. A smaller version of me in my head felt willing to obey...then Vaas-Thunder came along and kicked that fucker in the face with a pair of assault boots. I snapped myself out of the trance with a quick shake of my head.

Sorry, that happens. Just, erm, ignore it if an image of my flanks appears in your vision, that's just me getting comfortable. Onwards!

After a rather confusing flight, all three of us managed to escape the caves relatively unmolested (except my poor mind. Nightmare was practically licking my brain and confusing the poor sod), before a swift journey over Canterlot was made. A short landing on the bedroom balcony later, several minutes of shutting the curtains, locking the doors, and soundproofing the room, and Nightmare popped out of my head, giving the feeling again and then turning into the smoke figure and lying beside Luna and I. More specifically, I.
"Well, that was a most interesting series of events." Nightmare said contentedly. She was barely visible in the naturally dark room. "Masters, I'd recommend taking a shower, since Thunder is covered in blood and [NO GORE TAG, CAN'T SAY], while you, Luna, are covered in dirt, soot, and some blood from Thunder."
"Good idea, Moony." I groaned, closing my eyes for a second and rubbing my face. "I'll probably pay the others a visit, just so they know I'm here. Plus, Lulu and I will need to tell them you're no problem now."
"I'm no problem." Nightmare said cheerfully.
"I know. Luna, you want to share the shower with your handsome stallion of a coltfriend?"
"Nah, I'm fine." she yawned, getting up and slipping under the bedsheets. "I'll just leave you two so I can have some rest. And while I'm sleeping, you have my permission to make love..." Light snoring was what followed. I chuckled at the last comment and sat up. 3...2...1...
"Would, erm, you like to make love?" Nightmare asked quietly, looking rather bashful about asking.
"Mmmm..." I looked her up and down. "Would it even be possible, you know, you being a sentient steam cloud?"
"Oh, that can be easily circumvented." Nightmare giggled. "I'll possess Luna again. When I'm in her body, I become the original me. You know, the one you defeated almost a year ago?"
"Yes, I remember." I chuckled, standing up. "You also possessed ME, as well. I don't remember that. Anyway, go ahead with the Luna thing. Just promise to leave her body again? I don't want to have to rut you out of it. I don't have the energy." Nightmare rolled her eyes, evaporated, and shot up the bedsheets, and into Luna's open mouth. After a few seconds, nothing happened, before Luna opened her eyes, they were glowing a brilliant white, lighting up the room and making my adorable marefriend look terrifying. Slowly, she stood up onto the bed, tugging me back onto it with magic and pinning me down. She looked rather insane, so now I knew what I looked like.
"Well, Nightmare, it seems our planning paid off." Luna laughed. "Now we have him right where HE can't do much but feel what-" Luna's face contorted suddenly, giving the impression she was in pain, before her entire body exploded to transform into Nightmare Moon. She looked terribly imposing when you were in bed with her, I'll say that much. "-WE have been preparing him for for the past few hours. Thunder, this will be bordering painful on how amazing this shall feel. Now then: shall we begin?"
The tall black mare smiled seductively, and lunged upon me.

Author's Note

I'm Batman.