Tsuma Omoi, Netorase | Let me steal your wife's feelings

So there was a hentai I read a while ago called ' Tsuma Omoi, Netorase | Let me steal your wife's feelings.' When I read it, I expected the usual trashy NTR story, but it surprised me as it had a good ending. Then there was a good reason for why it was all happening. It got me interested in wanting to write something like that— 

The premise of the YC is in the hospital for an illness that was meant to be terminal, and their significant other has no idea what to do and feels hopeless. An unknown entity (Grim reaper) offers your characters SO deal if they can cast a curse on both of them. It will save YC's life. However, there is a condition that has to be met—

• For six months, YC must have sex at least once a month, or they will die.

• The second curse is that YC's significant other; they are not allowed to have intercourse for six months.

This is meant to be kept secret from YC, or the curse will lead to her death. We can discuss how it's going to happen, and plan it out.