This story includes bondage and lesbian tendencies. If this offends you turn back now.

There it was again, the persistent stare at the back of her neck. Robin didn't mind so much at this point. Tharja had been with them long enough for her stalking behaviour to become a normal occurrence and, in the end, she was mostly harmless. The white-haired strategist was freaked out at first, during the first few weeks of working with her, but by now the hex maker had become almost predictable. She was probably ducking behind the supply boxes in the corner of Robin's chamber, watching the young woman lounge on her bed, her robes and trousers set aside, reading a book. Tharja's presence didn't make her uncomfortable anymore, in fact it was strangely comforting, knowing that she wasn't alone, should something happen, yet left to her literature in silence. Finally, she decided to close the book and sit up, facing the boxes.
"Hello, Tharja.", she said with a smile.
The dark-haired mage rose from behind the supplies, as predicted, her curvy figure clad in tight, revealing clothes as always, only somewhat covered by her cloak.
"You noticed me.", she commented flatly. Robin thought to detect a note of disappointment in her voice. "You're getting better."
"You flatter me. I only got used to it, that is all." Robin sighed and put the book aside.
"I see... maybe I should hide somewhere else next time.", Tharja mumbled to herself, her eyes darting across the room before fixating on the other woman. A slight grin crept across her face as she climbed over the boxes and headed for the bed.
"Never mind that... do you know what day it is, Robin?"
The strategist put her index finger to her chin, frowning.
"I don't know... did I miss something important?"
"It's... Fell Day. It's a Plegian holiday."
The dark mage reached the bed, sitting on the edge, not breaking eye contact.
"That's... odd. I never heard of that day before.", Robin inquired, raising an eyebrow.
"It's rarely celebrated today... but I like it. It's a day where we give presents to... comrades and... friends." Tharja's grin wavered a bit at the last part, as if the words left a bad taste in her mouth. "Regardless... I have a present for you~" She reached into her cloak, producing what looked like a tiny black belt with a silver buckle and ornaments.
Robin eyed the supposed "present". She knew her too well. The collar was cursed. Some kind of curse or hex was placed on it and the whole Fell Day act, not only poorly made up but also poorly executed, was a ploy to make her wear it. Tharja still grinned at her, awaiting her reaction.
"Wow... thanks." Robin hesitantly took the gift into her hands. "It's... pretty."
It really was. The small ornaments were carefully crafted and surprisingly detailed motives she recalled from magic formulas, although none of them seemed applicable to anything Tharja might have it do, at least none of them raised any red flags for Robin.
"I'm glad you like it.", Tharja whispered, sliding closer, "Why don't you put it on?"
What could it possibly do? Robin eyed the collar carefully and looked back up at Tharja, who had gotten pretty close already. For a moment her eyes wandered, eyeing Tharja's barely hidden body. What would she possibly do to her? They were on the same side, that much was clear by now. Tharja could have betrayed them on many opportunities and she never did. She was strange, yes, and creepy, absolutely, but she did not want them any real harm. Whatever it was the collar did, in the long run it couldn't hurt to humour her.
And it was pretty. So very pretty. The more she thought about it, the more she wanted it around her neck, wanted to wear it, at least put it on once, at least once to know how it felt...
Before she realized, she had already opened the buckle and put the leather around her neck. She was unable to stop now, already under some kind of curse, fastening the buckle again, pulling the collar as tight as possible without having it be uncomfortable. A low chuckle came from Tharja.
"Oh, yes~ I knew it, it suits you so well~" She crawled onto the bed, straddling one of the younger mage's bare legs. "It looks so good on you, Robin." She chuckled again, gently but with surprising strength taking Robin's shoulders and pressing her against the back wall at the bed's head.
"I... thank you..." Robin looked up at her, blushing at the sudden contact. "Tharja, what..."
"Just so you know", Tharja interrupted with a wider grin than before, her eyes sparkling, "this collar is hand-crafted and only responds to me. As long as I don't permit you to, you won't take it off or cast any magic."
"Wh-what!?" Robin grabbed the buckle but it wouldn't open, no matter how hard she tugged at it. Tharja didn't let her try long, though, as she gently grabbed her wrists and held them above her head, pressing them against the stone wall. Her face moved closer, the tips of their noses now just an inch apart.
"You. Are. Mine.", the dark-haired woman whispered softly, her gaze now longing and excited. Robin gasped quietly and widened her eyes in shock. She tried to push her away, but her wrists wouldn't move.
Tharja's fingers moved down, running over Robin's bare arms like droplets of cold water. Robin's wrists would still not move, something invisible but immovable holding them in place at the wall.
Her first reaction was a small panic, pulling at her arms and trying to move her leg away from under Tharja, turning her head and attempting to conjure up magic, all of which contributed to nothing but fruitless struggle. The realization sank in that she had foolishly fallen into an obvious trap and was now utterly helpless.
"You are so adorable... Robin..." Tharja opened her cloak and tossed it off the bed, revealing all of her scantily clad figure before leaning in again and taking Robin's cheek, turning her head to make her face her. "My cute little Robin~"
Before Robin could think of any words of protest, the mage had leaned it, breathing a kiss onto her lips. Her other hand wandered down, stopping at Robin's chest, cupping her modest breast.
"Gah! Tharja, no... D-don't do that!", the white-haired girl protested with a gasp, blushing furiously, continuing to struggle pointlessly.
"Don't do... this?" Tharja faked a puzzled expression, cupping both breasts now, massaging them. Robin gasped and tossed, failing to hold back moaning. "Why? You seem to like it, Robin~"
Robin pressed her lips together, trying to silence herself, but the moans kept breaking through.
"Robin needs it, right~? Robin needs me to pleasure her. Robin wants me to~"
Tharja's hips started moving, her crotch rubbing against Robin's thigh. Her hands moved down to her victim's belly, slipping under the thin shirt. Her nails started lightly scratching over her skin, making the hexed woman wince and giggle.
"Oh~? Is my Robin ticklish~?"
"N-no!", Robin quickly blurted out, "No, not at all!"
Tharja's longing expression returned to the mischievous grin as she quickly ran her fingers over Robin's sides, making her giggle and shift, her eyes watering as she tried to hold back something else now. It didn't take long until she burst out laughing, tossing about in her invisible restraints, trying to move her torso away from the soft fingers.
"N-no! Tha-Tharja! S-s-sto-stop!", she begged in between gasping for breath, "P-please, no! Sto-stop it, plea-please!"
Tharja kept going for another minute or two thoroughly enjoying Robin's thigh rubbing against her crotch as she struggled, before she stopped, keeping her cold hands at Robin's side, waiting for her to stop laughing and regain her breath.
"You... are evil.", Robin finally huffed, relaxing a bit for the first time after putting on the cursed collar.
"That I am.", Tharja grinned back, sliding her hands upward again.
"W-wait... Tharja... I haven't..."
To her surprise, the hex maker listened to her request, stopping just at the base of her bare breasts under the shirt.
"Robin hasn't what?"
She blushed harder, turning her face away.
"Robin hasn't been touched like this yet...", she admitted quietly.
Tharja paused briefly but then grinned again.
"That is even better~ I will be the only one who ever pleasured my Robin~"
She grabbed hold of Robin's breasts again, gently massaging them. She leaned in closer again, breathing against her love's lips.
"I will be gentle for you~", she whispered, "Just this once."
She kissed her again, leaning into it more this time, pushing her tongue past Robin's soft lips, met with surprisingly little resistance. She licked Robin's tongue, convincing her to take part in the deep kiss.
One of her hands moved away from the strategist's chest, instead brushing past her belly again before reaching down into her panties. A protesting moan was all the resistance Robin cared to express before fully committing herself to the kiss again.
As Tharja's fingers moved faster in and out of her captive, she also starting moving her crotch faster against Robin's thigh, both women moaning ever louder into each others mouths until, with muffled screams and shivering bodies, they reached the climax together.

"Oh, Robin, there you are!"
She looked up from her book to see the blue-haired princess approach her.
"Oh, Lucina. Something I can do for you?"
The swordswoman smiled at her.
"We're waiting on you for dinner. Lost track of time reading again, did you?"
"I must have.", Robin laughed bashfully, "I honestly didn't notice how late it was."
They both turned into the hallway Lucina had come from and slowly walked towards the dining hall.
"Have you seen Tharja around, by the way?", Lucina started again, "It looks like no one saw her all day."
"Oh, I was speaking to her just last night. It looks like she's a bit tied up, since she massed up some kind of hex. I don't think she'll be joining us today."
"Oh, I see... That's a pretty collar."
"Thanks. It was a gift."