Despite not leaving until the unfilmed Christmas special, Peter Capaldi claims to have already filmed the 12th Doctor's death, saying, "I did it the other day. It was a strange day. It was explosive. He goes out a fighter." How this will play out remains to be seen. There could be a David Tennant-style thing where he holds the regeneration in for a long time. Or perhaps the regeneration is stopped by something, only for him to get killed again in the next episode.