Half

Cozy Glow found her in her study atop that same tower where Twilight Sparkle had once lived, muzzle buried deep in an old tome that glowed with magical energy.

"Golly, that looks scary," she said, leaning over to glance at the diagrams on the weathered page. “What’s it supposed to do?”

“I think it’s a summoning spell, but I’m not sure for what,” Luster Dawn replied. “I was hoping to figure out some way to bind whatever would appear, but no luck yet.” She looked up at Cozy. “Did you meet with Flurry?”

“Yup.” Cozy pointed at the book. “Is that thing dangerous by itself, or is it just the spells inside I need to watch out for?”

Luster glanced down at the book, looking surprised. “Oh, yeah, don’t worry,” she said, finally seeming to notice the glow. “That’s just a light spell I was using earlier today before the sun came up. Forgot to turn it off.”

“Great!” With a sweep of her hoof, Cozy knocked the book out of Luster’s hooves, then grabbed her face and pulled it towards her for a passionate, possessive kiss, not giving the unicorn time to recover from her shock.

They stayed like that for several minutes, drowning in each other as Cozy held Luster tight, refusing to let go even when she tried to pull away.

Then, magic enveloped Cozy and flung her across the room, pinning her tightly against the wall with enough pressure to drive the air out of her lungs.

“If you did that out of guilt because something happened between you two, I’m going to flay you alive,” Luster said calmly, fixing up her mane with her hoof from where Cozy had mussed it up during the kiss.

Cozy struggled to reply, but the magic choking her was too tight, and it was all she could do to stay conscious as the life drained out from her small body.

Her mouth opened and shut several times in a feeble attempt to beg for air. Luckily, Luster was well-versed in reading her intentions and released her, the pegasus mare gasping to the ground as she coughed and sputtered for breath.

“Stars, you’re paranoid,” Cozy eventually gasped out, glaring at her.

Luster stared at her impassively. “It’s a good trait to have when you've got a teacher who'd entomb us both in stone for what we truly are. Besides, how else would I have noticed the princess sneaking around the palace during this visit?”

“Well, you still got it wrong in the end,” Cozy said smugly, picking herself up off the ground. "Flurry wasn't looking for some dark spell to give herself ultimate power or claim dominion over the world." She trotted over to Luster and booped her on the nose. "She was looking for something to impress her crush with."

Luster raised an eyebrow at that. "Oh? Yeah, that changes things." Her head tilted towards Cozy. "And were you able to wheedle out the name of the pony she's head over heels for?"

Cozy's lips turned upwards, but her response was nearly hissed through gritted teeth. "It's you, you dummy."

Luster reeled back as if struck. "What?"

"Yeah, and you'd better be thankful I didn't stab her the moment she told me," Cozy replied, reaching out to cup her chin in a vice grip. "I think that deserves a bit of a reward, don't you think?" She forced her lips against Luster's once again, this time meeting far less resistance. “Anyways, since I’m so smart and clever, I managed to improvise,” she added after the kiss.

“So you were successful after all?" Luster asked, an eager smile spreading across her muzzle.

Cozy glanced at the clock. Fifteen minutes after what would have been high noon over Canterlot. “I think we’re about to find out real soon.”

As if on cue, there was a frantic banging at the door. "Luster Dawn? Luster Dawn! I need to speak with you immediately!" A familiar voice called out.

"Well, are you going to talk to your new future marefriend?" Cozy asked, at once smug and coldly furious.

Luster hesitantly opened the door. "Princess Flurry Heart, please come in."

Flurry didn't even spare a glance at Cozy, her eyes gazing rapturously upon Luster as she crossed the room in a flash of teleportation, prostrating herself before the unicorn. "Oh, Luster," she whispered, her eyes tearing up as a rictus smile stretched across her lips. "I can't hold it back anymore or else I'm going to burst. I'm in love with you, and I'm yours forever."

Luster raised an eyebrow towards Cozy, but otherwise remained composed in the face of this confession. "Oh?" she simply asked. "And what gave you the courage to confess like this?"

"A spell," Flurry said, still staring at Luster. "I found it in the restricted section of the Archives." She conjured up a notebook and floated it towards Luster. "I’ve adored you since the first time I laid eyes upon you, and I will follow your every command, my lady.” Cozy snickered as Luster professed this information.

“Be quiet for a bit then while I confer with my, ah, associate,” she said quickly, casting a privacy spell around herself and Cozy, her eyes scanning the copy of the ritual from Sombra’s journal.

“Not the spell we agreed on, but I figured this one would work too,” Cozy explained.

Luster’s brow furrowed. “This is one of Sombra’s, and invokes a lot of mind magic, but its true purpose is very deeply hidden.”

“It’s half of the spell he used to take control of another pony by instilling total devotion in them.” Cozy smirked. “It’s missing the half that inverts the effect.”

“I see.” Luster grinned. “So, Princess Flurry tried to bind my heart to her will, and bound herself to my servitude instead. Clever.” She leaned over to kiss Cozy, but pulled back. “Probably best to not test the limits of that devotion yet. At least, not until I can reinforce it properly.”

Flurry stared at the two of them, confused but obedient. The love that shone in her eyes was almost blinding, and made Cozy sick to the stomach to think about. How pathetic.

Luster broke the privacy spell to address the captivated princess once more. “I accept you as mine, Flurry Heart,” she said, and Flurry’s expression lit up into one of pure ecstasy at the declaration. “For now, I want you to keep our relationship a secret, as well as the fact that Cozy is my associate. Act the way you normally would, and don't raise any suspicions.”

“Anything for you,” Flurry replied with a vigorous nod. “I am yours, now and forever. Until the stars themselves burn out.”

“Well, for now, you should get back to wherever you were before and clean up any evidence of the ritual,” Luster commanded. “I assume you came here as soon as it completed?”

Luster nodded.

“Clean it up, then, and then come back in an hour,” Luster said, a clear dismissal.

Flurry bowed in deference, then teleported away.

“Well, that was rude of her,” Cozy commented with a huff. “Didn’t even bother to say hi to the mare who helped her out.”

“I wonder why,” Luster replied dryly, turning her attention back to her lover. "You did good today, Cozy." She reached over for a quick nuzzle. "I'm really happy that you're continuing to be useful."

"I'm happy you're happy," Cozy said, frowning. "Just remember who actually loves you and has failsafes in place all across Equestria in case she disappears."

"Aww, are you jealous?" Luster asked, grinning. "Don't worry, I'm not going to throw you out now that I've got a powerful alicorn at my beck and call. I’ve got plans for you."

"Good." Cozy allowed Luster to kiss her as she spun her own threads of machinations. "I have plans for you too. We're gonna do great things, the two of us, especially now that we've got a princess under our hooves.”

Luster nodded. "A new age for Equestria, and for us."

And so, on this fateful day, the two lovers kissed again, affirming their true love for one another.