When the drive for revenge ultimately leads to the death of Character A, his final words are a long withheld admission of love for his partner, Character B. As the life drains from Character A's body, Character B holds him and tries desperately to stem the flow of blood, to save the other man's life but he was doomed to fail. For years the two had worked together, constantly antagonizing each other as they both tried desperately to fight the pull towards each other. As Character A breaths his final breath, Character B realizes the truth, that they are soulmates and now he is just half a person. The loss hits Character B hard, hard enough to drive him to a dangerous foe. A deal with a demon sends him back in time to a point years prior in Character A's life before his family was murdered and he started down his doomed path of revenge. Character B was given that one chance to save not only his soulmate but also himself because if he fails, Character B will loose his soul and be consumed by pain and darkness, he will become a demon.
(Supernatural, time-travel, soulmates, SLASH. Characters A and B are part of a group that hunts down and eliminates supernatural threats, whether they themselves are supernatural beings or not. The line would involved a bit of bouncing back and forth. We can start when A dies and B makes the deal or start when B arrives in the past, or something else entirely. I was thinking that B could either pose as a teacher when A is in high school or we could say that A is a few years older and B poses as a fellow college student. If A is underage, then nothing sexual would happen in the past but the connection would still be there. Once B’s time in the past runs out, then we return to the ‘present’ and see if B succeeded in giving A a reason to turn away from his hunt for revenge.)