Cass had already teleported the rest of the crew off the ship when she encountered the Eighth Doctor, whose TARDIS had just materialised onboard. Recognising the Doctor as a Time Lord, Cass chose death over involvement with the Last Great Time War, as she considered the Time Lords as no different to the Daleks, despite the Doctor’s protestations that he wasn’t part of the war. The Doctor refused to leave her side even when it meant he would also be killed in the crash. While her body was recovered by the Sisterhood of Karn and they were able to revive the Doctor, her injuries were too severe for even their Elixir of Life to heal.
Her death inspired the Doctor to finally take part in the war. He claimed her bandolier as a reminder of what he would be fighting for and chose to regenerate into a warrior after the Sisterhood of Karn revived him long enough to select what his next incarnation would be like through their elevated knowledge of Time Lord science. (The Night of the Doctor)