(This may have a few spoilers) I had read a novel years ago where the MC is reincarnated because he was killed by a beggar that his family had allowed to dine with them and then it turned out the beggar was an assassin. Then the MC is reincarnated to another world but I can't seem to remember much else other than him being gifted a pendant on one of his birthdays. Then the family is attacked and then the pendant explodes and sends him to the underworld and all that is left of him is a crater and a purple flame where he had lain injured. Edit: I remembered that he also had nightmares of the killer every night but he didn't remember his past life at all except for that part. Please notify me if I didn't make this post correct as this is my first post, thank you
Then he gets like 2 harem members in that underworld arc. I know the novel, but can't remember the name.