I'm trying to add a bit of story to this riddle/puzzle, so blame me for trying! I hope you like it, might be a bit rough at first, it's the first time I'm doing this myself. I enjoyed making it so good luck!
You play a new character, whoever you want to be, but a superhero! You are a new recruit to the Avengers. Director Fury just found out his CORE is missing. All of the Avenger-team members are missing, missing as in on vacation or on a mission. As you are a new member of the team since today you are going to have to help Fury with this problem.
At the Avengers Headquarters
Director Fury calls you to his office and so it begins:
Fury: Hello new avenger, I'm director Fury.
You: Hello director Fury.
Fury: Come with me, it's urgent. This will be your first mission.
You walk to an office nearby where all camera footage is being kept.
Fury: The CORE has been stolen by one of our avengers. I believe he's working with Loki.
You: Is he still kept captive?
Fury: Yes, he's still in our dungeon. The thief didn't have much time to free Loki. I had a chat with Loki and he confirmed it's one of our avengers, sadly I believe him. Here's the footage of me chatting with Loki.
And so he shows you the video.
Click the video starts playing
Fury: What is so important right now that you have to tell me?!
Loki: But Fury, don't be so mad, I know your CORE is missing...
Fury: How would you know that?
Loki: It was my plan after all, only the thief who stole your CORE betrayed me.
Fury: Why would I believe you. And if so tell me who did it!
Loki: Well, ... one of your Avengers. he said with an evil grin
Fury: I don't have time to play games Loki, tell me.
Loki: Yet the thief did not free me, I still prefer him or her having the CORE over you. You don't like games Fury? Let's play one.
Fury: I don't have time for this...
Loki: Destruction is what I'm after. Remember this Fury ' +8+15-18-18+2-3+6-5+5-1-11 ' got it? Enjoy playing the game ha ha ha ha ... .
Click the video stopped playing
The video stopped playing.
Director Fury addresses you once more.
Fury: Son, as I don't have time to play games, you have to figure out what he means.
You: I will do my best. Do you have any idea what these numbers mean?
Fury: If I did I wouldn't ask you. You better figure it out!
So this will be your first mission. Figure out which Avenger stole the CORE.
Enjoy the game!