Looking for a fantasy novel where the main character returns from the future

Question

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?

Answer 1

First I wish to apologize I was able to take a look at my library and was able to identify the novel while also noticing several misidentifications I posted in the question. it is the novel Death Was But the Beginning by April O'Malley Summary:

They say that when you die, your whole life flashes before your eyes. Well, after being captured by Melephar’s servants to be sacrificed on the god’s altar, I can only concur… and I must say it isn’t a pretty sight. As everything comes back to me in a snap, from the murder of my older brother to my coronation and the decline of our kingdom, all I feel is bitterness and regret. How I wish I could do it all again but better this time. How I wish I could go back in time and prevent these catastrophes from happening! As the servants slit my throat and complete the ritual that is to grant them great power, something goes wrong and it’s my wish that gets granted instead. Next thing I know, I wake up in my old bed, an innocent child. After recovering from the initial shock, I decide to make the most of this second chance and save the kingdom by any means necessary. But there is much to do, and a little kid is not expected to make any grand decision or wage any wars. If I want to succeed, I will have to remain in the shadows. I will have to put on an act, play dumb in public and hunt down the sect that brought our demise when the night falls. To make matters worse, I cannot behave too suspiciously because I suspect someone is watching me. Hopefully, although I am but a boy now, I have a lifetime of experience to draw from. Let’s hope that will suffice because I know for sure I won’t get a third shot.
Quotes: When they’d began this little venture – now the largest underground organization ever to exist in the 12 Kingdoms

His brother, Zandheer Khan, was due to be assassinated in his own wedding feast with Princess Searshe, heir to the throne of Lundenwick – their marriage was meant to solidify alliances and mend the tattered relations of the two kingdoms; it also helped that the two were madly in love with each other. This would not come to pass – in his old life at least – when his brother was found with his throat slashed open, his blood running red through his bed sheets. Princess Searshe – being the only other person in the room – was accused of murdering her husband and executed the next day.