There's no real need to do this. I'm currently writing a fantasy series and I don't have an introduction. Instead, I've begun by introducing an important character connected to magic (this is in the first chapter), describing only his appearance and not connecting him to mysterious doings, so as to keep the reader interested.
Instead of introducing an entire world of magic to begin, I slowly provide information related to it as the series progresses. Again, this has the reader wanting more.
The main problem with an introduction is that your reader may skip it, as this is usually little more than an information dump. If your reader skips any part of your book for this reason, they will become confused (although that will be the reader's fault, the reader will still put down your book).
But if you feel you need an introduction, then go for it. Just make sure to keep it as interesting as the rest of the book.
Also, in addition to Lauren Ipsum's answer, I advise against a 'previously' intro. Again, information dump. What you might want to do instead is, for the first chapter (in every book of your series except the first), you can talk about what's happened in the previous books and connect it to what's happening in the new book. You should only retell the most important parts of the plot, to prevent reader confusion (for those who haven't read the previous books) but at the same time keep the reader interested, which is very important.