Everyone else has already suggested stuff I would, so I can only give آپ this oneshot manga fairly like Black Butler called Vartrag Tale.
It's about a boy about Ciel's age who develops a fascination with a pretty demon lord as a child & has to decide if he's willing to make a huge sacrifice.
Mamiya Oki's art is fantastic.
link There's also the supernatural Saiyuki gaiden-like series "Ja-Dou" that she did all the art for, but not all of them have been scanlated yet. There's like at least 7 books.
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