How many premises have you read that have had the words "harem", "fantasy" and "reincarnation" in them? Probably a hell of a lot of them.
More commonly known by its Romanized Japanese name, Katahane no Riku, Chronicle of Barusak is the antithesis of the harem reincarnation fantasy genre and is said to be comparable to the first novel of Arifureta before it went bad.
The premise is about a pair siblings who get reincarnated into a harem game. The guy, Rook, was fated to become the MC of the game. Whereas the girl, Riku, was to become one of his harem members.
This and that happens and Riku gets betrayed by the humans and defects to the demons. Joining their army, she ends up becoming a ruthless slaughtering demon out for human blood. This is the story which follows our MC, Riku, and of her plight.
Now before you go and say that she's just like any other weak-willed heroine driven by revenge, let me just say that this is tagged a tragedy for a reason. ;)
Now go and enjoy the harem destruction goodness!
P.S.: Yes, there are two MCs. There is Rook, the MC of the game world, and Riku, the MC of this story.