beta
I'm sure its manga with one shot BL.
The uke was reincarnated in the book, since he knew everything he was hailed as a prophet. But after the book is done, he basically lose his use. His fiancee broke up with him since he can no longer predict, etc etc. Then that's when the prince/hero came in ? (don't remember if its a prince or hero, the hero is basically yandere) and (had sex) took him in his place.
Camping here
found it while cleaning history lol
https://www.mangago.me/read-manga/substitute_mobs_cannot_escape/
Actually good for the MC now those little shits finally have guts to take a move, knowing the MC can be taken away any time