Just wondering.....Would demons be able to reproduce with humans like vastaya did?

I'd assume demons can't reproduce at all considering they're ancient spooky beings that are essentially immortal.

Sure why not? If you believe, anything can happen. Plus demons are cool. I don't judge.

######ᴾᵉᵒᵖᶫᵉ ʲᵘᵈᵍᵉ ᵐᵉ

It is safe to assume yes. Every fantasy story has allowed it. It would likely be difficult but could occasionally happen. But think of the neat cross breeds we could have.

Your friend must be writing an Evelynn fanfic. :3c

These spiritual creatures, known as the Vasthayshai’rei, welcomed the human refugees and eventually produced what Runeterrans now refer to as vastaya

It would depend on if the Vasthayshai'rei are actually spirits from the spirit realm or if the word "spiritual" is just being used to describe anything magical.

It also depends on wether spirits and demons are related, but I think they are.

I wonder how a hybrid baby would look like