HELP

I'm finding a BL manhwa, it's about a vampire who is after a holy knight's blood. They met in an alleyway. The knight was originally from a magical place, he was after a dragon that was endangering the princess of that place. But then he found out the dragon and the princess was married to each other. He got this curse that is stuck forever but his mind is off it because the vampire was always annoying him. He is a professor in a school btw. The knight tried getting rid of the vampire by paying vampire hunters but failed. That's all I remember, please help me I've been finding it for days.