Thank you!Yes, this is a good sentence to introduce Prince John, especially if you are going on to say more about him and why he was evil.
Robin Hood's money and land were stolen by an evil man, Prince John.It would be possible to use 'the' even if this is the first time you mention him, and you still have to describe him.
Robin Hood's money and land were stolen by the evil man, Prince John.
Without more context, I prefer 'an', but if I saw the rest of the paragraph I might prefer 'the'.
You do need either 'an' or 'the'. You can't omit the article altogether.