When you create an antagonist, consider how they will behave during conflicts.
Powerful antagonists might also be protected by a “plot armor” that prevents Player Characters from fighting them unless they have a specific item, spell, or other assets at their disposal. Generally you don’t want to overdo this — one or two such “tricks” over the course of the entire campaign are more than enough.
A Villain’s entrance should be an intense moment that sets the tone for their role in the story. Think of it as a video game cutscene: you get a few seconds to show your audience what this Villain looks like and hint at the kind of forces and weapons at their disposal — soldiers, forgotten technology, powerful magic, or a healthy dose of supernatural strength.
During your preparation stage before a session, it may be helpful to focus on the reveals key moments — you have to condense everything important in a couple of sentences. It should be something like this:
Approaching with heavy footfalls, the figure rests a hand on a sword the likes of which you’ve never seen... a delicate gesture, but her grip is steady.
“It appears we are here for the same reason,” she says. “Unfortunately, I cannot let you have that which I need. Stand back, and no one shall be injured.”
With a few words, you have communicated that this is likely a proud and honorable adversary who is also very determined. That sword of hers? That’s bad news!
"When you chose to fight not to protect your people, but to prove them wrong... that is what made you a villain."