Luna caught up with Discord before he left the palace. "Discord!"

"Come to see me off?" he asked teasingly.

"I would speak with you before you leave," she said. "This is a matter of importance, pertaining to the war."

He raised a dark eyebrow. "Hmm. And not something you wanted to share with me in front of Celestia."

"Yes," she said firmly.

"Very well, then." He snapped his claw, teleporting them both to her bedchamber. "I imagine we should be able to talk here."

Luna looked around herself guiltily. "Discord, this is an unwise place! It's not seemly for you to be in my bedroom; what if the servants overheard and told Celestia? What would she think?"

"Well, what she had better think is that neither she nor anypony else can own me," Discord said coolly. "And if she doesn't trust me not to go sneaking behind her back to seduce her little sister, who, and I will be frank here, might as well be my own little sister, without at the very least warning her that I plan to do it, then she doesn't deserve me." He leaned forward. "I have no interest in the palace's stupid rules of propriety. I put up with them while Starfire was alive out of respect for the pony who rescued me from slavery and treated me like his own son. But he's dead now, Celestia rules in his place, and I will do what I find convenient. If I want to have a conversation with you in your locked bedroom, I will. It's hardly as if you could ravish me against my will, after all." He smirked.

"I do not think anypony would be concerned with the thought that I might ravish you."

"Yes, well, anypony who thinks it might go the other way around is hilarious. And also a frog, if I catch them making slanderous claims like that about me. I don't ravish ponies and I'm not going to start with my lover's little sister. Not even if you say please, Luna, so for both our sakes, maintain your dignity and restrain yourself from begging for me. I know I'm much too handsome to resist, but be strong for Celestia's sake."

Luna sighed. "Can you never be serious?"

"Yes, I'm pretty sure that I'm able to never be serious. Will I never be serious? Well, life's full of surprises, but right now, it seems more likely than not."

"I need to speak with you, regarding the mission my royal sister has sent you on."

"You can start the speaking any time, you know. You don't actually have to waste both our time moaning in disappointment that I refuse to ravish you."

She smacked him with her forehoof. Discord just grinned. "Be serious! I know my sister princess has ordered you to the dragon lands. And I know the true meaning of 'Cry Havoc.'"

Discord went still. "Spying on Tia and me, were you?"

"Your dreams are locked to me... but Tia's dreams have never held any secrets, not from me. I know she has sent you to bring chaos to the dragons... and not only that. I know she has instructed you, not to focus on killing their bodies but on destroying their souls, bringing them madness and breaking them."

"And?"

"And, I want to help."

Discord laughed. "Luna, Luna, Luna. As powerful as you are... your father was just as powerful, and battle-hardened, and he fell. What makes you think you'd fare any better? Or that I can safely take any ally into battle with me? Chaos has no friends. I go alone precisely because what I will do to the dragons, I couldn't keep from doing to anypony who went with me."

"I'm no foal, Discord! Don't treat me as if I'm a foolish filly still!" Luna snapped. "I am Luna Dreamwalker, Princess of the Moon, heir to Queen Imbrium the Queen of Storms, and my power is not to go with you in body into combat. My power is to walk in dreams. Dragons dream as much as ponies, or even more; they require more sleep. I want you to show me the chaos that will rip their minds apart, so that when they retreat from you into the sanctuary of dreams, I shall follow them and give them no respite. I shall bring to their sleep what you give to them waking. And I can do it here, from the palace, for that is my power, Discord. I am the Mare of the Night, and when the moon is up, all those that dream are subject to my will."

He ran a claw through the thick mop of black mane on the top of his head, grinning. "Oh, my. Luna, I never knew you had it in you. Such ferocity. And here I was thinking you were still just the little filly I used to tickle."

"My father is dead. I am my sister's sole heir. And we are at war. I cannot afford to be a little filly anymore."

"Hmm. Well, it would be remarkably hypocritical of me to deny you. Celestia is sending me on this mission, where Starfire never did, because he saw me as a colt, and she sees me as a stallion grown enough to face danger alone. And yet, she makes the same mistake with you that the king made with me." He suddenly grabbed her cheeks, pulling them outward to expose her teeth. "Let's see..."

Luna yanked her face out of Discord's grasp. "Discord! What are you doing?"

"Looking a gift pony in the mouth, of course," Discord said. "You can never be too careful."

"Be serious for once!"

"I am." He stood up straight, his full height looming over her. "Luna, nopony can stop you from walking in dreams. You'll try to do this whether I train you or not. And it's true that dreams are the purest chaos most who aren't me can ever perceive. So perhaps you can handle this. But Celestia will be angry with me for exposing you to horrors that threaten your sanity; Starswirl has always said that ponies who try to be chaos mages all die young, go insane, or go insane and then die young. Can you, truly, do this? Without losing your own mind?"

"I do not mean to become a chaos mage, Discord. That, I will leave to you. But I am the Mare of the Night, the Princess of the Moon, and I walk with my subjects in the darkest of places. I must be strong enough to bear horrors, for who else will ever guide and protect ponies through them? Celestia is no dreamwalker and would only try to dispell them with light; but the light can never last forever. Only darkness brings peace and refreshment to the mind, only darkness brings the warmth and safety of sleep, and bringing light to the darkness would only ever destroy the peace the darkness had come to bring. And you? You would just laugh."

"I might be persuaded to help a pony or two deal with something horrific," he said, pouting. "I wouldn't just laugh. All the time."

"But 'tis your job to bring horror to our enemies, Discord, not protect our own in the valley of darkness. That is my burden to bear. If I cannot wield the darkest of dreams against Equestria's enemies, how can I protect my subjects from them?" She reared up and put her hooves on his chest. "Discord, let me help. You are chaos, I am darkness; we wield the powers Celestia dares not, we can stoop to the levels she cannot, so we can bring her home a victory over dragonkind and keep her pure. She thinks she protects me, she thinks I am yet an innocent filly. But I am a dreamwalker. I have not been innocent in many long years."

"Can't be that many long years, you're not that old."

"I'm old enough!"

"Tell me, Luna." His head snaked down and around her, to speak to her face even as she leaned up against his chest. "Do you think King Starfire would be proud of you, of us, if we break dragonkind with madness and chaos and dark dreams, if we make their days terror and leave no peace to their nights? Do you think he would be glad of our victory?"

She looked away in the other direction. "He would despise what we have done," she whispered. "He would call our work abomination and repudiate us, perhaps strip me of my crown and cast us both from the castle. He might even despise us."

"And does that make you have second thoughts?"

Luna dropped to four legs and whipped around to face Discord's head, wings flaring outward as if she meant to fly at him. "They ate him!" she screamed. "My father was brave, and noble, and a shining star in the firmament, ethical, pure and true, and the dragons ate him alive! And I will tear their souls from their minds, blacken their hearts in despair and take all the evil in the world into myself if I must to make sure that does not happen to my sister!" She breathed raggedly, tears forming in her eyes. "Equestria needs a good ruler. A ruler for peace, for love, for harmony. A noble, ethical queen to lead our people into the light, a shining inspiration to all ponykind. And for Celestia to be that queen, I must be the Princess of all dark things. I must make the night my weapon so that my sister's sun should never need to burn red."

"Then we are in agreement," Discord said softly. "Because I will turn the world upside down, sideways and inside out to keep her safe. I think she's less pure than you think she is—"

"I do not need to hear about my sister's love life," Luna said hotly, cheeks burning.

Discord chuckled. "Now, why did you think I was talking about that?" he asked cheerily. "It doesn't matter to me how perverse ponies might think our love life is; I don't personally think anything done with consent and love is ever wrong, so why would you think I would confuse lack of sexual experience with purity? I mean that Celestia created Cry Havoc. She asked me to go alone to dragon lands, and break them, to unleash chaos unrestrained until she should cry halt. And when I asked her if I could focus on breaking their minds rather than their bodies, she agreed with me. She told me that was better. And... she has seen me in moments of madness. She's seen some of what I can do when I don't restrain myself, more than you have. I think she knows exactly what she has unleashed. That is not the act of a pure, sheltered, noblemare; that is the decision of a cold and ruthless queen. One I am proud to serve, and one you should be proud to rule beside."

Luna nodded once. "Then she should understand why I must do what I must do."

Discord laughed. "You are such contradictions, you two," he said. "The sun is hot and the moon is cold, and yet you have such fire and she can be like ice. I suppose that's part of what I like about you both, the chaos in the contradictions."

"So you will do it?"

"For a price. Show me how to walk in dreams, Luna Dreamwalker; the minds of ponies are closed to me, and they shouldn't be. The mind, after all, is the home of all harmony, and all disharmony. It should be my realm, but Starswirl was never willing to teach me." He smiled sardonically. "You want to know the truth, I think he's afraid of me."

"I know he is. You wield chaos. You repeated his words to me, ponies cannot do so without going mad. You may be a draconequus, but there's no lore to suggest your kind found it any safer to carry chaos in their hearts; we can only guess what it will do to you. Of course he fears you. I think... I think he fears me as well."

"Well then. He's trained us both, but likely held both of us back out of fear. Let's train each other. Show me the door into ponies' dreams, and dragons', and I will show you how to use chaos and madness to rend our enemies' souls and fling them gibbering into an abyss of lifelong insanity and torment. And then, proud dragons will bend their knees to ponies, at last. They think little herbivores will never defeat them, and they don't fear death; and they think I'm some perverse, impure hybrid, a parody of a dragon, of diluted and inferior blood, less than they are in every way. Let us teach them, you and I, how wrong they are about us, shall we?"

"Yes," the Night Mare said with soft ferocity. "Yes, Discord, we shall."

Author's Notes:

A lot of fans have speculated that Discord was responsible for Nightmare Moon. I think he was too, but not necessarily in the direct sense of preying on Luna's mind; after all he was sealed in stone at the time. Instead, I think he encouraged the side of her that eventually became Nightmare Moon, not for malicious reasons but for the same reason he lost his own grasp on whatever empathy and kindness he ever had.

In my universe, the road to hell was paved with the very best of intentions.

No one uses "thou" because I don't feel like writing it; it's a difficult construction to master. And Luna is not yet ruling as a diarch, so she does not use the royal we. I tried to make her language (and Celestia's, in the fics of the same time period that Celestia is in) sound somewhat archaic and overformal and High Fantasy, whereas Discord *usually* sounds more generally casual no matter what time period he's speaking in, but even he occasionally slips into High Fantasy type dialogue.

Sharp-eyed viewers will note references to Discord having dark eyebrows and a mane on the top of his head, not just the back of it. It's intentional. He was younger then. He also doesn't have a goatee, but that doesn't come up.