The English second-person pronoun used to be more complex than the current you and was declined as follows: thou and ye were the subjective singular and plural; thee and you were the objective singular and plural; thy/thine and your were the possessive singulars and plural. Already beginning in Middle English, the plural forms slowly replaced the singular forms and eventually merged into what we use today.
From late Middle English to early Modern English, the singular forms—thou, thee, thy, and thine—were saved for informal, familiar, and condescending situations (akin to the modern uses of tu, du, and tú in French, German, and Spanish). Thou even doubled as an insulting verb meaning to refer to someone as inferior: “Thou viper, for I thou thee, thou traitor!” (Sir Edward Coke to Sir Walter Raleigh, during the latter’s trial, 1603). But this historical nuance has been lost to modern speakers, who mistakenly believe that the archaic forms were formal or honorable, possibly due to their use in the King James Bible, whose writers may have wanted to portray god as familiar and close to man rather than an unapproachable creator.
Avoid the kitsch habit of using the archaic forms to emulate formal and aged speech. A favorite of fantasy writers, I have never seen it used correctly.