Doctor Who: Every Version Of The Doctor Explained
9. The Dream Lord (First Appearance: 2010)
The Dream Lord was a manifestation of the Eleventh Doctor's darkest impulses and thoughts, who was brought to life by psychic pollen trapped inside the TARDIS time rotor.
The Eleventh Doctor was able to defeat him by identifying which of the Dream Lord's realities were real, and which were tricks.
Played by Toby Jones, this darker version of the Doctor shared much in common with the Valeyard, and they could potentially even be one and the same.
The Valeyard was supposed to come into being between the Doctor's twelfth and final incarnation. The War Doctor, plus the Tenth Doctor's extra regeneration in The Stolen Earth, meant that Matt Smith's Eleventh Doctor was actually the Thirteenth Doctor.
It's terrific head canon, even if that was never the original intent.