In the original series, the Valeyard is a future incarnation of the Doctor, but twisted into an incarnation of all of his evil and dark inclinations. He puts the Sixth Doctor on trial (in the Trial of a Time Lord) in an attempt to steal the rest of the Doctor's regenerations for himself.

There is also a rather brillant Big Finish Doctor Who Unbound AU story (He Jests at Scars) with the Valeyard and Mel Bush. But if it is the Valeyard, I will be disappointed if they don't use the original actor though :(