you were at your own apartment, feeling bored, you searched for something on the shelf and found a magic spell book, you knew it wouldn't work, since magic isn't real, you looked at the cover page, and it said "how to summon a demon" you curiously opened it up, and saw a demon prince, so you followed the instruction, and you know it will not work, you tried it, you said the word, and a demon was summoned, it wasn't just an ordinary demon it was a demon prince, you summoned along time ago maybe 9years ago when that happened, up until now he was still rude and cold, but he was submissive to everything you said, and from that day on whenever he was hungry, he would feed on your blood

time skip

you were just doing some paper works when suddenly shinji barged into the room looking hungry and he said

"master....i-i....I need your blood..."

you nodded and gestured for him to come, you tilted your head making your neck exposed to him, and he went closer to you, and fed on your blood, you never ran out of blood anyways and just let him drink as much as he wants