The demo version often lags slightly behind the main Steam version, but purchasing the game on Itch.io gives you a Steam key, as the full version is only downloadable on Steam.
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Hi Nishazzar! We want to do other platforms, but I can't really tell how long it will take to support Linux. So far we've heard that Book of Demons runs smoothly using PlayOnLinux, so you can try the demo and see if it works for you :)