How legal are fan games?

I want to make a sekiro fan game, specifically, I want to recreate the tutorial (and maybe other sections, but that's for later) in the style of a lego game. Since sekiro is a game, and this is a fan game, I thought that might already be an issue.

Where are the lines between legal and not legal, assuming I publish this game free on itch.io?

EDIT: Ey, thanks for the replies. I'll be sure to make my own idea instead