So, I'm creating this race known as the Alptraum. They're shadow creatures who can take a human form, but they can only come out at night because the sun burns their eyes. However, I'm having trouble specifically regarding their diet. They do eat raw meat, however, I'm debating if they also feed on nightmares or the fear nightmares induce. If they feed on nightmares, they would replace them with sweet dreams. On the other hand, if they fed on fear, they would be able to distort those sweet dreams and turn them into nightmares. Anyway, what do you think?
It sounds to me like you're asking the wrong question. The question you should be asking is what this race is supposed to do in the story. What purpose do they serve and how do they help your overall theme. Once you answer that, your questions should be answered.
@Kallisto is on the right track here, I think. Do you want them to be scary? Then it seems like they should probably cause nightmares, not good dreams. On the other hand, a spooky creature that also has the ability to give you good dreams is pretty interesting. Are these complicated, compelling creatures? Maybe the latter option would give them a little more depth, make them more nuanced. On a horrific third hand, an evil force with the ability to make you feel good about the weird and awful thing it's doing to you could be even more spooky.
Why are you asking me? I'm not writing your story. I don't what would and wouldn't work. I don't know what your themes are or your characters or even your plot. And judging by how you're answering this, I don't think you have a clue either. Don't you think you should, should have an idea of a story first before you figure out how all the pieces fit?
Um...izzybot isn't the OP, and wasn't talking to you. Those questions are for the OP's consideration.
Second @Kallisto and @izzybot There's not a lot that can be said here without you giving us more information about what you want these creature to do in your story. We don't really have any idea why you even included them in the first place. You haven't told us what your story is about either. Is it a comedy? Adventure? Action? Drama? Light and quipy or darker and pondering? It's good that you haven't info dumped a whole bunch of irrelevant details, but you should give some context on what you're trying to do here.
Oh gosh, I'm so sorry izzybot. I was not paying attention to who was saying who. Thanks E Equstris for pointing that out.
Admittedly, I'm not currently planning to write them into a big story, though that might happen in the future. To explain, I simply like to create other races and characters that aren't related to the stories I'm writing because it's fun and helps to relieve stress. However, I was leaning more towards them feeding on nightmares, and your idea has made me decide to go along with that @izzybot , especially the horrific third hand. Just imagine someone entering your mind while your sleeping and take something away from it. Though you mind not have wanted that thing in the first place, it's still really creepy.