In Universe Edit
When the Eighth Doctor crashed on Karn during the Last Great Time War, he knew that he would need to become a warrior in order to end the Time War, and the Sisterhood of Karn helped him control his regeneration into the War Doctor (played by John Hurt).
Out of Universe Edit
Interviews with Russell T. Davies, as well as information in spin-off media such as "The Gallifrey Chronicles" (BBC Eighth Doctor Adventures novel #72), and the "behind the scenes" features in "The Flood" (graphic novel), made it clear that the Doctor who fought in the Last Great Time War was the Ninth Doctor (Christopher Eccleston). It was Steven Moffat's original intention that the three Doctors who appeared in "The Day of the Doctor" should be the Ninth, Tenth and Eleventh Doctors, with Eccleston fulfilling the same role that Hurt ultimately did.
However when Christopher Eccleston was unwilling to reprise his role as the Ninth Doctor, Moffat changed his plans, and came up with the 'War Doctor', a forgotten incarnation between the Eighth and Ninth Doctors. John Hurt agreed to play this character, and we got "The Night of the Doctor" and "The Day of the Doctor" the way we did, with John Hurt as the 'War Doctor'.