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I think it can work if done well. I read both Sci-Fi and fantasy and enjoy both so I'm perhaps your target audience as I see no reason that the two could not be mixed if there is a point to doing ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/24592 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I think it can work if done well. I read both Sci-Fi and fantasy and enjoy both so I'm perhaps your target audience as I see no reason that the two could not be mixed if there is a point to doing that. As long as the story supports the need for it and it's done in a logical and believable fashion then it should be fine. You could also explore the effect of the discovery of real 'magic' on the advance sci-fi people. Maybe initial denial and attempts to scientifically explain it followed by a split in the culture caused by part of the population simply wanting to believe in the 'magic'. That might give your Elves an embedded force sympathetic to them and allow them to eventually win even if they were not initially superior to the high tech people on the battlefield. On the other hand if you try and cram both into a single novel just as a means to try and double the potential audience then it will probably fall flat.