[quote=Magic Magnum]
Who/What's Valeyard?
[/quote]

:lol I need to save that. 

BUT ANYWHO, VALEYARD:

[hider=Classic spoilers]The Valeyard was essentially an evil duplicate of the Doctor. Which... sounds kind of lame at face value, but [i]holy shit[/i] that's just a terrifying idea when you think about it. It's like... the Doctor is already scary when he wants to be. And now he's evil. [i]Welp that's something![/i] 

It probably helps that he didn't [i]look[/i] exactly the same as the Doctor, either. He was sort of described as a "splintered regeneration", so he looked no more like the Doctor than any of his previous regenerations did. This allowed him to keep his identity hidden for quite a while before it was revealed just Who he was (hehe). Also, when he [i]was[/i] revealed, the Master teamed up with the Doctor in order to defeat him, because his logic was basically along the lines of "If only one of the two survive, the Valeyard will wipe the floor with me, but the Doctor I at least stand a chance against in the future." So um... yeah he's pretty damn powerful.

Also interesting to note is that, during the climax of that episode, the Doctor and the Master had seemingly destroyed the Valeyard. [i]However[/i], at the very end, it was revealed that the Valeyard survived, in a rather "dun dun dunnn!" sort of fashion. [i]However[/i], despite pointing out to us that the Valeyard was still alive, implying that he would later return, he actually never did. He hasn't been seen in another episode since. 

[i]Also[/i] it was implied (sort of... honestly they never explained it well) that the Valeyard was sort of created either at the end of the Doctor's regeneration cycle, or [i]after[/i] it, implying that he'd broken the 13 lives rule, which somehow resulted in the Valeyard's creation. This, plus the fact that Moffat kept hinting at the possibility of more Time Lords being in Capaldi's era, as well as the show becoming more like the Classics, all led me to believe that the Valeyard's return was a strong possibility. o.o [/hider]