<human>: My BF and I have been together for about eight months. We go to the same boarding school and are in the same year. 

He really likes Slenderman, and for Halloween, he dressed up as it. He makes a very convincing one because he is tall and thin. Once, he tried to play a Slenderman video game with me, and ever since, I've been terrified of that creepy looking thing. 

Last night, I was returning to my dorms after a late club meeting. It was dark out and I was practically the only person on the street. Out of nowhere, something pops out from behind a tree. It's him, dressed in his costume (but of course I did not realize that at first). I might add that the costume was super realistic. He had put on a white pantyhose over his face so it actually looked like he had no facial features. I completely freaked the fuck out. 

He then laughed and took off the mask. I was crying at that point and told him that it wasn't funny. He said, "It was funny to me", and teased me for being so scared. I stormed off back to my dorm. 

The next morning, he acted as if nothing had happened. I asked if he was going to apologize and he was like, "Why should I? It was just a joke." He had told his friends about it and they all thought it was absolutely hilarious.

Now I'm afraid of walking back to my dorm at night, all thanks to him. He even has the nerve to accuse me of being "too uptight". How do I let him know that his prank actually wasn't harmless, and that he's being inconsiderate by brushing aside how traumatizing it was?

Can you condense the text into a few key points?
<bot>: BF dressed up as Slenderman in order to scare, even though he knows how afraid I am of it. He refuses to acknowledge how terrified I am and says that I'm overreacting.