Person A had a former love, but for whatever reason, A was the cause of their death. A dreams about it, but instead of the first person, they saw B die instead. (BONUS- B assures them it was a dream, and that they were never going to leave.)

http://otpprompts.tumblr.com/post/132583053570/person-a-had-a-former-love-but-for-whatever

This is literally a canon Parabellum scene. Go ahead and write it, dude. (How about using it as a fill for [30-dreams] 30. dream/nightmare?)