The dark entity chuckled softly as her host lashed out at her mate as if on cue. She couldn't take all the credit, but most of it for enraging the princess against her father, anger made anypony rash and thoughtless and Nightmare Moon took full advantage of her grasp on Luna's emotions, turning them against her, curdling her anger in a stew pot of seething fury.
For months since her imprisonment at the hooves of the elements of harmony, she had floated on a sea of nothingness, death. She had laid so peacefully, everything forgotten until a surge of emotion brought her back to life inside the mind of her host. She awoke to find the princess embracing a mare and felt a surge of jealousy and resentment. Here she was bound in this body, being yanked back like a fish on a line from her dead slumber into the body of somepony happy. Happiness was the one thing the spirit hated. She could never have that in life, taken into Father's care alongside Chrysalis, she had been raised in chaos. Like Sissy, she was devoid of any happiness or joy under the cruel reign of Discord and she had learned that love and hatred were the same things, while infinitely different. She had never felt love, and hated all who had experienced and relished in it with a passion that burned like the fires of Tartarus.
As foals, she and Chrysalis constantly fought over Father's attention like canines and felines, until Chrysalis discovered her power to feed on emotions. Exercising her new found power, Sissy destroyed Nightmare, eradicating anything capable of love as well as her life. Nightmare's hatred brought her back into the world as a malevolent spirit, wreaking havoc on anypony's lives she came upon in vile spite. For centuries she drifted on the winds as a shadowy mist, destroying lives in her wake out of complete hatred for the world.
She had chanced upon the moon-princess in the haunting of a house, grief and loathing seething from the delicate princess. Nightmare could care less about whence the emotions came, she barreled into them full force and took over the body of the alicorn. Taking control of the ravaging hatred, she gripped the handle-bars like a bicycle and pushed the vehicle into the upbringing of eternal night, justice for her host, ecstasy for the symbiote. Grief and sadness flooded the land in the eternal night until her host's sister banished the pair to the moon.
For a millennium Nightmare stewed her host's hatred, driving her into insanity so that the symbiotic entity could sit back and watch her monster destroy the happiness of all. Things had worked out better than expected, on the night of her return nopony save one could even remember who she was, and even that one had no idea how to stop her, until the end. Her defeat was an unexpected drawback, the result of her letting her host fight the battles. Most decisions were hesitated upon, and the alicorn was nowhere near as violent as Nightmare herself was, the esper would have butchered the annoying ponies without hesitation. She had barely hung onto the princess' psyche as the strange magic hit her, trying to drive her out, but the effort of holding on had "killed" her immortal spirit, making her comatose until Luna's joy brought her vengeful soul back to life.
Now she relished in her victory, ruining her host's happiness and joy, yanking her into the pits of despair as she so enjoyed doing. Nightmare kissed the walls around Luna's mind, preying on her weak desperation to gain entry. She very early did, but at the last moment, the princess slammed the gates shut. Nightmare tried again and this time was met with more resistance, her quarry more alert than before. Her mind whipped out at the spirit trying to get rid of her, but Nightmare took advantage of the opening and darted in under the defenses.
"Hello again my precious." She cooed tenderly, much like a lover would.
"Get out of my head you bitch." Her host snapped back.
"Ooh touchy touchy. Do you treat everypony this way? No wonder nopony loves you." Nightmare engaged tactics she had mastered over centuries with the alicorn, she knew every weakness in her mind.
"Twily loves me, as does Tia, Cadance, and Twily's friends in Ponyville."
"Not anymore, not after hurting Twilight so. Dear Tia hates you for having sex with her student and Twilight's friends will not forgive you for hurting her like that. Even your own Father hates you for loving a mare, he abandoned you and forsook you."
"Liar!" she protested halfheartedly. The snake had bitten, the venom was flowing through her mind, seizing her emotions and turning them, slowly, painfully, driving every ounce of emotional agony Nightmare could draw.
"You know I am right. I was before, when our dear sister cast you out."
"Sissy forgave me! And she apologized, telling me how much she regretted the decision!" Luna fought back.
Nightmare recoiled as the nickname, her same nickname for Chrysalis, but she brushed it aside and dove back into the fray, or rather argument. "And do you really believe her? Her student was right there and she does have a reputation to consider. Lies are easily made, especially when one is desperate."
Luna shook her physical head, pushing it into the pillows as she fought with everything she had, but the malevolent ghost had her in a vise, tightening her grip with every gentle word.
"We both saw the old Celestia tonight, cruel, abusive, and less of an angel than she would have everypony think. She keeps it hidden behind a facade, but heat the mask and it melts, she is just the same as before. Your sister hates you, she doesn't love you, you were always in her way. She-"
"Shut the buck up! Get the hell out of my head and just shut up!" She screamed both mentally and physically, making the bed shudder under her powerful voice.
"You know I'm right my precious." were the last words Nightmare had for her host as she returned to her silence, but remained in Luna's warring thoughts.
******
Nightmare drifted about Luna's conflicted mind, observing her poison as it flowed over the patterns in her head. Doubts, gnawing worry, self-uncertainty, all the tools that composed Nightmare's deadly brew of hate turned emotions one by one, giving the ghost a sickening pleasure. She arrived at a front of reasoning that held firm, Luna had convinced herself that Celestia truly did love her. Her symbiote wheedled its way into the wall of conviction pushing back the wave of doubt and tried a new tactic, one that the ghost had dealt with in herself numerous times and had experienced its effects personally.
She carefully injected a single thought,"Nopony could love me after all that I have done. Nopony could completely forgive me for that."
If Nightmare had a face she would have smiled, if she had possessed a mouth she would have laughed, the wall crumbled under the weight of the thought. It made all her resolve and determination fade like darkness before the dawn. The analogy gave Nightmare yet another thought to distribute, a familiar one with Luna. She inserted the old thought, "Tia's dawn will always put my moon away. She will always be better than me."
Luna's will gave way with a splash and puff of shadowy smoke as Nightmare rushed into the frail mind of the princess and grabbed hold of her entire body. In the corporeal world, Luna's body vanished in a puff of glittery mist and the smoke settled down into the form Midnight Moon preferred, hard armour over her breast and a silver helmet over her head, interrupted only by a single horn. Her mane billowed like a sheet of smog flapping in the wind, not really a part of her black body. Her tongue rubbed against four razor-sharp fangs that replaced a normal ponies canines and the teeth just behind them, and the lips that covered the pointed teeth stretched into a smile of victory and cruelty.
******
Nightmare floated through the windows of the Celestia's throne room in the form of her swirling mist. The sentient smog settled on the floor and coalesced into the form of the evil incarnation of Luna. Looking around, she saw the room was abandoned and reverted to her living fog.
The fog settled in the gardens and formed their being again. Nightmare spotted Celestia looking about panicked, but the alicorn did not seem to notice her audience.
"Discord! Show yourself!" She called to the empty air.
"No need for me to. Turn around, you have a guest who would very much like to see you."
The sun-princess turned quickly and nearly doubled over when she spotted Nightmare Moon. "Luna, you know I hate that form, it brings back too many memories of... before."
Nightmare reverted back to Luna's actual shape, but held her sour expression on her face, enjoying the charade and the opportunity it gave her to hurt Celestia. "Watch and learn my precious." she whispered softly in her mind to Luna.
Celestia walked closer, looking scared out of her mind. "Luna, Discord is free! We have to get the Elements of Harmony to imprison him again!"
Nightmare looked at the oblivious princess with distaste written plainly across her face. "Including Twilight? I'm sure you will have a hard time with that. I broke up with her, so are you happy now? You ruined the happiness of not one, but two ponies. How does that feel, Angel?" She spat "Angel" as an insult, Celestia's face twisting in displeasure.
"Look, now is not the best time for that, we have to deal with Discord."
"You deal with him. I don't owe you anything." She hissed, turning her back and walking from the garden. Celestia trotted to catch up.
"Look, I'm sorry I overreacted. I just-"
"I don't want your pathetic apologies. Go buck yourself."
Celestia stopped and looked at her in horror, "Why would you say that?"
The impostor chuckled and changed form to the one she preferred, "Just a little family reunion, and I would enjoy seeing my father again."
"Wha-" was all the alicorn could get out before a lion-paw came to rest on Nightmare's shoulder, materializing out of thin air with the remainder of its owner's body.
"Good to see you again too Nightmare. Thought you would never get a hold of Luna, and look how perfect the timing was! It was perfect for a little family get-together!" The draconequus chimed happily.
"What in-"
"Take a good look Celestia, this is what happens when you can't accept your own sister. Family comes first, and I had a much better father than you and poor Luna ever did."
Nightmare Moon filled her horn with magic and slammed the white alicorn against the palace wall with enough force to make the walls crack. The alicorn was knocked unconscious and dropped to the floor with a satisfying thud making Nightmare smile in pleasure. She was enjoying her vengeance, sweet and juicy like the meat she hadn't eaten since she had her own body so many years ago.
"Well Night, you are a lot meaner than you were the last time I saw you."
"If you knew what Sissy did to me then you would understand." Nightmare responded, looking at the prostrate princess before her.
"Hmm, won't ask. C'mon, let's go make some chaos, and while we are at it, maybe we can get little Chryssy to join us. I do hope you don't mind."
"I don't." was her only response as the father and daughter vanished into thin air.