Nightmare realized his mistake the moment he had settled in his office. The child was sleeping on the sofa in his office, curled up in the hoodie he'd given him with his newly blotted cheek pressed against an arm rest as he slept. The mistake wasn't taking the child with him, no he wouldn't have left him without a fight. His mistake was not realizing sooner that he was in no way prepared to care for a child of about five and in fact had no knowledge on how to raise said child. No one had raised him and Dream, they had been fully functioning for the most part upon their creation and only needed to be taught the nuances of social interactions when the village was made around their tree.
Nonetheless, Nightmare found himself the guardian of a five-year-old abandoned AU Sans who had absolutely no knowledge of anything. The child hadn't even understood what clothes were before he'd explained them to him when giving him the hoodie. He also couldn't continue calling him "the child" for the rest of the time he was here and would need to think of a name for him. It had angered him that the creator of the child's AU hadn't even had the decency to leave him with the name of Sans, his name was just blank.
He looked over to the child and thought back to when he had first seen him. The way he had been sketched by his creator was almost like he had been drawn with flowing ink. He had seemed to stabilize once Nightmare brought him to his castle with the only sign of the previously sketch-like lines being the black patch on his right cheek.
"Hmmm. Ink perhaps?" Nightmare hummed thoughtfully to himself. He would have to see how the child liked the name when he woke.
Nightmare spent another hour or so reading at his desk before he heard the child stir on the couch and sit up with a yawn. He looked over and smiled slightly as the child rubbed at his sockets before looking around in confusion.
"Did you sleep well little one?" He asked, pulling the child's attention to him. They blinked for a moment before seeming to remember where they were and how they got there.
"Yes. I've never had to sleep before. Is sleep normal?" He asked curiously, getting up to move to the front of Nightmare's desk.
The child was so small that he barely reached the top of his desk and rose up on tiptoes to peek over it at him. Nightmare fought another smile as he nodded in response.
"Yes, sleep is normal for all those in the multiverse. Since your AU was outside of the Multiverse it may have allowed for you not to require sleep while inside." Nightmare explained thoughtfully, "But enough of that for now. How do you like the name Ink?"
The child blinked at him in confusion before taking a thoughtful look.
"It sounds nice. Are you naming someone?" He asked, making Nightmare smile in amusement.
"You, little one. If you like it, Ink will be your name from here on." He replied. The child blinked once more before lighting up in excitement.
"I GET A NAME!?" He exclaimed happily, making Nightmare chuckle at him.
"Yes little one, you get a name. I think Ink is quite fitting." He said as Ink bounced on his toes happily.
Yes, Ink was a good name and now Nightmare's main concern was how he was gonna raise the child when he had duties as a guardian that couldn't be neglected for long. Something to worry about later though. His attention turned to Ink as the child excitedly began asking questions about the different things in his office. For now he would focus on learning about Ink and see what the child needed to be taught and go from there.