Yandere!Under verse x Reader

Memories and Nightmares

Y/n immediately recoils and stands up, knowing that voice. Even if corrupted, they still remember their brothers voice.

Y/n, hesitantly:...Nightmare?

Nightmare, growling: Obviously! Who else would it be?!

His tentacles lash angrily and Y/n's eyes shadow, their tail lashing. Nightmare growls, waiting for a response.

Nightmare: really? I haven't seen you in hundreds of years, and this is the thanks I get for seeing you again?

Y/n's eyebrow twitches and the room gets colder, ice spreading throughout the room. Nightmare raises an eyebrow and tenses slightly, looking around the room, before looking back at you.

Nightmare: What's wrong? Did I upset my baby sister-?

Nightmare's sentence gets cut off by Y/n lunging at him and hugging him, much to his surprise. Nightmare just stands there awkwardly, not entirely sure what to do. He didn't expect Y/n to hug him in all honesty. He thought they would try to attack or yell at him....but his train of thought gets cut off by feeling his getting wet and the sound of Y/n crying. Looking down, he sees that they are crying, clinging to his shirt.

Nightmare: uhh...you okay?

Y/n, through tears: i-it's been years you jerk! W-where were you?! I-I tried to look for you, b-but couldn't find you!

Nightmare sighs. He expected them to cry if he was being honest...but something felt wrong about this. He is the guardian of negativity, he's spread chaos and despair through the multiverse...so why doesn't he like Y/n crying? Perhaps a part of him still cares about Y/n, even if he won't admit it. He then hugs back, remembering the times when he would do this when Y/n was upset or had a nightmare when they were younger. Nightmare then decided to try and at least attempt to comfort his sister.

Nightmare:...I was doing my job as the guardian of negativity.

Y/n, sobbing: d-did you even attempt to look for me? A-and what about Dream? W-why can't I find him either?! D-did you even care...

Nightmare internally curses, trying to think of something to say, but all of his attempts just end up being bad. Y/n clings to him more, their wings flapping in distress.

Nightmare:..Truth be told, I was too angry to look, and Dream is....somewhere...but yes, I do care.

Y/n nods, still crying. Nightmare then sighs, and uses his magic to eat Y/n's negativity and send them to sleep....again, but this time, it's just a normal nap instead of being forced to sleep for a century. Y/n collapses, but Nightmare's tentacles catch them, and then he teleports them to Y/n's room, putting them on their bed and tucking in. Nightmare then crosses his arms,bone brows furrowed in thought.

Nightmare:...why do I even care? I haven't cared before, so why am I caring now? I hadn't even thought about them until I sensed their negativity and came over to get the negativity to get stronger...Why was them crying different for me? I haven't cared when anyone's cried before, so why do I care now?

Error:...Nightmare

Nightmare sighs in annoyance, looking at the glitchy skeleton.

Nightmare: Error. Why are you here?

Error: Visiting. Why are you here?

Nightmare, raising a bone brow: Am I not allowed to visit my sister?

Error narrows his eyes and sends strings at Nightmare, who dodges and slices the strings with his tentacles.

Error: They never mentioned having brothers.