I believe that Steven Moffat now ties the record for most incarnations of the Doctor created under his tenure as showrunner; at three. Eleven, Twelve (who seems to be getting excellent reviews from the "Deep Breath" previews) and the War Doctor, as played by John Hurt.
For those of you who haven't seen "The Day of the Doctor" yet (and why haven't you), the "War Doctor" is the incarnation of the Doctor between McGann and Eccleston; the one who fought the Time War and who as a result is the Doctor's biggest secret. A grouchy and stern old man who renounced the name 'Doctor', he is also a deeply compassionate individual who wants to end the war... although he is horrified at what he might have to do to end it.
After his three appearances in the TV show, the War Doctor is now making his first appearance in Doctor Who's vast expanded universe in an original novel by George Mann, who has previously written an Eleventh Doctor novel. From this, it's clear that he's more than just a one-trick pony.
Where we're at
Set towards the end of the War Doctor's long life (he ages a great deal between "Night of the Doctor " and "Day"), he doesn't have a regular companion and nor does he want to take the risk of acquiring one.
Arriving on the war-ravaged human planet of Moldox, the Doctor teams up with a local woman to stop a Dalek plot to win the Time War and gain control of all history.
- Cinder. The "guest companion" in this one is a Dalek hunter only known as 'Cinder', who lost her family in the Dalek invasion of her homeworld. She proves an able and fierce ally of the Doctor, who is willing to let her carry a weapon with her (provided she doesn't actually use it). Well written and very interesting.
- The War Doctor. An excellently portrayed character both on screen and in print, this Doctor combines a strong heroic streak with the irreverent attitude demonstrated through all his incarnations... and maintains his standards through all the horror.
- The Time Lords - we get the return of Rassilon as portrayed by Timothy Dalton and some extensive sequences on the Doctor's home planet, including one place I didn't expect. The rottenness in the state of Gallifrey is visibly dripping in these scenes and some very dodgy stuff is done.
- The Daleks. Well developed, not prone to being defeated in stupid ways and their relationship with a certain Time Lord is well explained.
- We get one recurring character too many coming back.