It was either a litrpg or a fantasy novel. The main character is/was prince of the local kingdom and he and several others were sacrificed to a god by a cult. Rather then dying they return, to the past. One of them was an assassin for the cult, but because he returned to a time before he lost his wife, he decides to eliminate the cult members.
Previously (historically,) everything was ruled by one empire, but it disintegrated into kingdoms (there very well could have been four of them.) The cult was/would be responsible for continental war between the kingdoms.
The main character is now an overlord/leader of the underworld and has a female disciple (could have been a dark elf.)
These are some further points that may have been part of this novel:
- The main character's brother is supposed to marry a princess of another kingdom.
- The bride was assassinated by the cult during their wedding night in the previous "timeline" but was saved by the main character in the current one as he kills the assassin in secret before he can hurt her.
- There is a caravan travelling to the wedding reception carrying gifts, which is joined by a monster hunter.
- The society of monster hunters predates the dissolution of the Empire, also monster hunters are legendary for their fighting prowess.
- While travelling through a mountain range, the caravan is attacked by a dragon, but the main character intervenes and defeats it.
- I believe that the main character also has a sister who has been made the lord of a city so he gifts her a magical book filled with knowledge.
Does anyone have an idea which novel it could be?