Tanya the Evil

While this is a niche character type, there are probably some tips to making it work.  I mean, advice on any subject is helpful, right?  We’ll see how this goes because 7 might be hefty challenge on this one.  Wish me luck.

1. If you want the villain’s origin to be a surprise then you have to make sure they come off as a local.  The clothing has to be from that world.  The way they talk and their knowledge needs to be near perfect.  I say near because there should be some subtle hint that there is something off.  This way, the reveal has impact.
2. In regards to the hint, I don’t mean to make it clear they are from another world.  I mean to have there be a sign that they are hiding something.  This is done for all kinds of secret origins, powers, and plans.  Maybe they get flustered in a certain situation or there is an aspect of the world they hate more than one would expect.  For example, the character could really despise a certain food that’s popular, which wouldn’t raise many eyebrows.  Yet, you learn it reminds them of the only food their mother gave them back in their world and they have issues.  Keep in mind that it doesn’t have to be that silly.
3. Having them act superior to the locals isn’t necessary, but it would make sense given they are a villain.  This works primarily with a character who is out to destroy or conquer.  If they come from a world of technology then they could easily see magic as inferior, which is different than a hero who sees it is as equal or better.  It doesn’t have to be blatant either.  Perhaps it could be a simple coldness towards the fate of the locals as if they are tools or not even real.
4. The moment the reveal is made does NOT mean you suddenly introduce a bunch of other world tech.  One will wonder why the villain was keeping all of the weapons and advancements a secret.  You could solve this by saying they were designing everything prior to the reveal, but that can bring up another question.  Why would they be a clear villain if they didn’t have their superior armory ready?  Each item needs an explanation for its existence too because you can’t just have computers appearing in a magical world.
5. A question can come up on if other characters know this secret before the reveal.  It depends on the situation though.  Having secondary villains makes sense if they see the main one as a deity or superior.  They might want access to the tech too.  On the other end of the spectrum is them learning the truth and seeing their ally/leader as an outsider.  I would say the level of xenophobia in the villain ranks will be a big deciding factor here.
6. Not every villain from another world knows how to build a nuclear bomb . . . or even a firearm.
7. The way they came to the new world should be considered and can be a key factor in them going evil.  If they were died and then reincarnated then they may be traumatized by remembering their own demise.  This could lead them to believe the new world is fake and they’re dreaming while in a coma.  Those that are simply plucked and transported may decide they are chosen to be gods or kings, which could tie into them having no power in their previous life.