>>297081 (OP)
Now, imagine the husband is also immortal; except he cannot die by any means what so ever. He and his waifu will both live for a long time, but eventually she will die by some means that he cannot control. So as a result he will try to control her life and ruin their love.
Imagine knowing that you could spend forever with your waifu, but one little mistake would kill her and spare you. One day she will die by some cosmic alignment. After all, probability states that given enough time and resources anything can happen.
immortality a la perpetuals in WH so the husband can still "die" but he won't stay that way. It's the first thing that came to mind with true immortality because Legion is such a good book