Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei suru anime will have original content.

On the official Twitter account for the anime adaptation of the light novels written by Rui Tsukiyo and illustrated by Reia, Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei suru (The world's best assassin, To reincarnate in a different world aristocrat), a statement was posted warning of changes to the original story of the work in its move to the anime adaptation. The commentary was written by the author himself, Rui Tsukiyo:

"The animated adaptation of Sekai Saikou no Ansatsusha has been approved! In order to improve the quality of the project, I would like to request the director to recruit talented people. I really hope that the final product will conform to my ideal of a dark aristocrat.

Additionally, for this production it has been decided not to directly take the content of the novel and adapt it to animation, but to go deeper into the work, deconstruct and re-create it. There are a lot of loose ends, but this is an opportunity to repair the plot and I am supervising everything. So, you can rest assured that it will be a faithful adaptation to the original concept."

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アニメ化記念コメント到着！
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原作：
月夜涙先生@Tsukiyo_rui

キャラクター原案：
れい亜先生@aosorayuri24

コミカライズ：
皇ハマオ先生@sumeragihamao

▼全文はこちらからも！https://t.co/8ueDHGjtld #暗殺貴族 pic.twitter.com/qvJI4qxnWn
— 『世界最高の暗殺者、異世界貴族に転生する』公式 (@ansatsu_kizoku) February 15, 2021

The series is scheduled to premiere in Japan next July. Meanwhile, Rui Tsukiyo, who is also the author of the light novels Kaifuku Jutsushi no Yarinaoshi (Redo of Healer), began publishing the novels through the independent service Shousetsuka ni Narou in July 2018. Subsequently, Kadokawa Publishing, through the Sneaker Bunko imprint, began print publication in February 2019.

Synopsis of Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei suru.

Earth's greatest assassin only knew how to live as a tool for his employers, until they stopped allowing him to live. Reborn by the grace of a goddess in a world of swords and sorcery, he has been offered the chance to do things differently this time, but there's a catch... They have to eliminate a super-powered hero who will bring about the end of the world unless he is stopped. Now known as Lugh Tuatha Dé, the master assassin certainly has his hands full, particularly because of all the beautiful girls who constantly surround him. Lugh may have been an unparalleled assassin, but how will he fare against enemies with powerful magic?

Source: Official Twitter Account