I read this years ago, so please bear with me 😅

The MC (I don't remember their original gender) reincarnates into a empress from (I think) a book they read. She's blonde and has red eyes, if memory serves. She also has a right hand man who has black hair. I'm pretty sure in the "original story" the black haired guy kills the empress after she becomes a tyrant.

Any help would be appreciated!

Sounds somewhat similar to Tensei Babaa wa Misugosenai!: Moto Akutoku Jotei no Nishuume Life

EDIT: Except the other way around. Empress is the one that reincarnates. Probably should have read your post more carefully. Oops.

~4400 manga completed
~1700 ongoing...

That's not the one, but it does look interesting, so thanks regardless! Lol

Probably The Rebirth of a Tyrannical Empress. Pretty sure this was novel reincarnation and has a male attendant that kills her in the original. It's been a long time since I read any of it.

This one was the one I thought too