In the IDW comic miniseries The Forgotten, the Tenth Doctor recounts to Martha Jones a story from the Eighth Doctor's participation in the Time War. The Eighth Doctor was imprisoned by a race of robots for several weeks on a planet in the middle of the war, before teaming up with a Malmooth fellow prisoner and faking his death in order to escape. It is revealed his capture was staged by him so that he could acquire the Great Key he needed to arm a modified De-Mat Gun that could be used to seal the Medusa Cascade. The Tenth Doctor further implies that the Eighth Doctor died, alone and companionless, at the end of the Time War.
Read more about this topic: Time War (Doctor Who), Time Wars in Spin-off Media