Many genres, especially fantasy, make use of anachronistic language—enough so that it’s worth taking a closer look at words like thou and thee.
Anachronistic language is, simply put, a style of speech that belongs to an era other than the one in which the book is written. While it can add a new dimension to your dialogue, use it with caution—it’s easy to abuse language we don’t see often (in prose, anyway).
Knowing how this speech was used when it was extant helps us apply it to our own writing. We can then go in and spice up our dialogue with the appropriate pronouns—we just have to be sure not to go overboard.