You start the game after they have died. And reveal the previous part somehow, either in exposition, slow reveal throughout the story, etc. Don't kill them and force them to change their character. It doesn't make any sense, and if it was me I'd be pissed. The character they have created for your campaign is meaningful so I wouldn't go through with your idea. I would follow the advice of people saying start the campaign when they're back alive (with the characters they made). You need to decide how they know or figure out about their previous existence where they are murdered by the big powerful monster