"I'm glad you visited today." Starlight brought her hooves together, a calculating smile on her face. "I admit, I'm surprised. No +1? No kid? I thought you were kinda attached to both of those."
"I let them have fun in Canterlot." Umbra's horn glowed as she summoned her book. "She's still learning stuff... and I learned stuff. You meet a Minuette?"
"Friend of Twilight's?" ventured Starlight. "About all I know about her. I see her around town once in a while..."
"That's the one." Umbra set the book down. "Now, I think we have business that's been waiting a while."
"Oh, I love where this is headed." Starlight rubbed her hooves together, smile turning malicious. "Rules, first to give up or pass out loses. Let's try not to lower the population of Equestria Wizards today, hm? Just some fun between peers."
"Those sound like reasonable rules." Umbra vanished, sinking into the darkness beneath her and popping free about fifteen feet away. "Shall we begin?"
"With pleasure." Starlight sent a beam of searing light, her sparkles dancing around the beam as it struck the ground Umbra had been a moment before, already leaping away.
The rocks it hit burst into flames and began to pop like popcorn in a cascade of puffs, but neither was paying attention, there was a battle to participate in.
"So..." Shifting considered her roomie. "I'm the mom right now."
"You are in charge," allowed Morning. "But you are not mom. You are mom's girlfriend, and maybe wife, later."
Shifting frowned at the idea. "And if we marry, doesn't that make me your mom?"
"No." No argument given, just a flat denial. "Are you strong?"
Shifting inclined her head. "In what way? I could pick up big heavy things."
"That's something, but not something that impresses me." Morning crossed her arms. "Mother is a creature of magic and shadow. I am a creature of shadow and nightmare." She flashed a smile with a predator's intent. "Impress me."
Shifting raised a hoof to her chin in a moment of quiet thought. "I tought your mother how to make magic things, so she could--"
"--Thank you." Morning inclined her head. "Are you offering to teach me?"
"You're in the middle of learning something," huffed Shifting. "You don't need more lessons with your lessons. How's your schoolwork going, by the way?"
"Almost done." Morning reached over to pat her book. "And at the requested pace. Time flies when you're enjoying yourself, and the nightmare lessons are quite enjoyable."
"Good. Good...." Shifting considered and perked suddenly. "How long did she say she was going to be gone?"
"Approximately 5 days." Morning perked an ear at Shifting. "Why?"
"Because I hear her." Shifting threw open the door and trotted out without a delay.
Morning blinked in shock. "Mother?!" She hopped to her hooves and scampered after Shifting. "You're home?!"
There was Umbra, coming in the hallway. Her face turned to a scowl on seeing Morning. "Didn't I tell you to stay with Shifting Prism? Where is she?"
Morning came to an abrupt halt, almost tripping. "She was just ahead of me, mother."
Umbra glanced over her shoulder, where no Shifting was. "I don't see her. Are you lying to me? I thought we had gotten past that..."
"N-no, of course not." Morning took an unsure step back. "I would never defy you, mother."
"Why am I wasting all these bits on this," grumbled Umbra, stomping right over Morning into the room. "I only have so many, you realize. You better be enjoying this."
"I am, mother." Morning hurried to catch up with her. "I am quite grateful. How did your visit go? Did you teach that pretender what true terror is?"
Umbra smirked maliciously. "As if you know."
"Mother?"
"As if you know," repeated Umbra, each word slow and precise. "You think you know, but all your terrors are up here." She tapped at her head. "You wouldn't even know what to do if they came out in person."
Morning trembled in place. What had she done to upset her mother? "I fear no terror." Her mother, perhaps, especially at that moment. "I will face the worst you have to test me with."
"What you fear most..." Umbra leaned forward, hooves clapped under her chin. "Is being shown you're wrong." With a rush of green flames, Shifting took the spot, smiling in the same predatory way. "Gotcha."
Morning blinked softly, unmoving, frozen. "Oh..." She crashed to her haunches. "I feel very stupid..."
"You're not. This is what changelings do." Shifting stepped down to stand. "I'm not mad at you. I hope you're not too mad at me."
"Why would I be mad at you?" Morning smiled sedately. "You have shown me a new fear, a novel horror. I will hold it preciously." She made as if to hug that ephemeral thing close. "This is a gift I will value."
Shifting considered the umbrum filly. "I'm still learning you." She reached out and wasn't resisted as she gently pet the filly. "But I'll keep trying. I know I feel like... extra, but I am a part of this family too, and I want to do my part."
Morning considered that with a curled hoof. "I understand that. Wanting to do my part is how I... I understand." The two met in a soft hug, though it was one that Morning broke first. "You want to be my mother? You have more to prove." She stood up and walked away at a relaxed rate. "But I will allow you to try."
"That's all I'm asking." Shifting moved instead for the kitchen. "Now what fearsome dinner are you looking forward to?"
The world slowed dramatically. Umbra flowed around the sparkling beam of pain, moving far faster than she usually did. The spell wore off an instant later. She wasn't Minuette, such tricks were draining, but it had worked. A second blast scorched her hoof, proving that Starlight was frighteningly good at sparring. "Blast it." She ducked under the third and sprang up in a twirling punch that cleaved through the space Starlight had been in.
"Woah ha," called out Starlight, looking way too happy about things. "Back off" She lashed out a hoof, catching Umbra and sending her flying in a twirl feet away. "You're going down." A sphere of power erupted up in time to catch and deflect Umbra's dark blast. "I practice this all the time."
Umbra scowled at her sparring partner. A pity she had nocreature to spar with. All her friends, aside Starlight, didn't want to do battle. "Dark has downsides." Her horn flashed brightly, that glow inverting, sucking light away from around and plunging the entire battlefield into darkness. She abandoned her physical form, becoming the lurking formless horror of the dark.
"Cheap trick." Starlight looked around wildly, her horn glowing to throw off light, but it was suffocatingly dark in all directions. "This isn't going to get you a win..." She fired wildly where she saw some movement, but no sound of pain came. Did she miss, or just see things?
"Why did you leave us?" Double Diamond emerged from the dark. "We wanted you, Starlight. Only you knew how to run things."
Starlight cringed back with a yelp. "Back off!" She zapped the pony, causing them to evaporate into mist. "This isn't fair..."
"We were surprised." Sugar Belle emerged with a haunted smile. "It happened so suddenly, but it wore off. Come on back. Nopony else is as ready as you are, Starlight Glimmer. Guide--"
She evaporated, magic slicing her in half. "This stopped being funny!" But she was breathing hard and scowling. "You won't shake me. Besides, this isn't magic."
"You just had to tell me the truth." Twilight appeared, wings flared. "If you enjoy controlling ponies... who am I to hold you back?!"
Starlight screamed and blasted the fake Twilight, but she didn't evaporate, instead blocking the bolt and turning it aside. "They clearly want you. Why were you hiding it from me?"
"That's sick!" practically shouted out Starlight. "I was wrong... They were wrong. Everypony involved was wrong." She sagged back, her fighting spirit having fled her. "I don't want that...Normal ponies don't want that."
Twilight advanced on silent and sedate hooves. "It's alright..." She put a hoof on Starlight's shoulder, eyes flaring with shadow magic. "It's okay..."
Shadowy chains wrapped around Starlight with unnerving speed, suddenly binding her so tightly she fell over. Twilight, returning with Umbra's visage, put a hoof on Starlight's nose. "I win."
"That wasn't even a little fair! We were supposed to match magics, not mind games." Starlight sniffed, eyes still stinging. "Not fair..."
"Shadow magic is mind games at least by half." Umbra waved away and the darkness fled, leaving them in a sunny field. "I've learned a lot of nice spells, but the not-nice ones are still there. Oh! You know I'm an umbrum, right?"
Starlight slowly rolled around, trying to get upright with limited success. "Don't quite know what that is... Still dirty pool."
"I am a creature of nightmares." She flashed a too-wide smile of sharpened fangs. "I can protect you from the night, or make you scream."
Starlight huffed. "Right... When do you let me go?"
"When you admit that you lost." Umbra nodded simply. "Then we're done for the moment."
"Ha...No." She burst with brilliant light, melting the shadowy chains away and letting her bounce to her hooves. "I'm going to tan your hide for what you did!"
Umbra squeaked and fled the incensed unicorn. Her plans had turned against her. Suddenly, everything was solid. She trapped her in a crystal block. Starlight sauntered up with a cocky smile. "What you get for thinking the fight's over before it is." She tapped at the crystal. "That was a real jerk move."
Umbra could not reply, encased as she was, just get out a muffled noise. "What was that?" Starlight raised a hoof to her ear dramatically. "I give up to Starlight who's way better at duels than me?"
Umbra rolled her eyes, but nodded, just faintly.
"Good." Starlight banished the crystal, freeing Umbra. "How did you even know about that stuff? I didn't tell you!"
"I didn't." Umbra stretched one limb at a time. "I let the nightmares in you out to play. I just played along, but I had no idea until it already happened."
Starlight hiked a brow. "Huh... Fine. I still won." She buffed at her chest. "But that was a good match, even if I still have to think a proper revenge for what you did! Um... So did you see that?" She began to color with shame. "Tell me you didn't."
But Umbra didn't lie. "I couldn't have predicted it, but I did see it..."
Starlight sagged. "You must think I'm a degenerate freak..."
"No!" Umbra pawed at Starlight's side. "You're an awesome unicorn, and still are. Seems, to me, that Equestria has no language to explain domsub relationships and how to approach them safely."
Starlight blinked owlishly. "Come again?"
"Domsub," repeated Umbra. "It's when you play at being the dominate or submissive part of a relationship. It can be entirely healthy, if done correctly. There are rules and ways to do it, and the one being dominated, the sub, is the one ultimately in control."
Starlight peered at Umbra skeptically. "If they're the one being controlled, how can they be in charge?"
"Because you care about them." Umbra tapped her hooves. "So you listen, closely. If they say the magic word, which you set ahead of time, or are otherwise not having fun, you stop. They call the shots, not you, the dom." She prodded at Starlight. "Which is what you are, little miss controller."
Starlight colored rapidly. "I'm getting better! No controlling ponies..."
"Hey, stop that." Umbra crossed her arms. "A little dom play is just fine, so long as you set the rules and obey the limits of your sub. The idea is to have fun."
"Fun?" Starlight shook her head slowly. "You need to explain this, all of this."