Congratulations! Your series has done so well it can afford a spinoff or side story. But using the main villain won't do; maybe this story takes place in another land or time period, or the main villain jsut wouldn't fit the story you're trying to tell. The solution is to make a new villain who will likely only be seen for the spinoff and never seen again afterward. Commonly a Video Game trope.
Hello, Unknown Troper. You'll need to get known to lend a hand here.