You were naughty kid... and Sans wasn't happy with your wishes and well... you looked at under Christmas tree and noticed present but as you touched, it suddenly explodes, blinding you and making you dizzy as you pass out with hearing of your heartbeat beating at 120 BPM

You woke up in your room, you were sleeping with Sans due to him being tired but when you look at your neck, you see a collar on your neck, a red one with soft white fur around it. As Sans woke up and suddenly tag it as collar tightens, making for you hard to breathe as your heartbeat increases due to restriction of oxygen due to collar as you try to gasp for air as your heartbeat reaches 150BPM as you can clearly hear how unhealthy in speed it beats. As Sans's holds you by your back as he rolls you to be face to face as he spoke in lazy, yet strict tone while his cheeks were glowing slightly blue from blush

Sans: heya, kid. you were naughty kid. so i decided to make you my elve.