The Demon Sword Master of Excalibur Academy is another pro-demon anime

Anime does like to come at you in waves of popular topics, and while isekai is undoubtedly one, it tends to focus on a you-or-I style character going somewhere fantasy and becoming mighty.

The demon ally story starts with the powerful demon and moves them into a (little) more mundane scenario.

The Demon Sword Master of Excalibur Academy (Seiken Gakuin no Maken Tsukai) Yū Shimizu and Asagi Tōsaka’s light novels are getting an anime treatment since they fit the demon scenario.

Yen Press have the English language rights to the books, picking up from Kadokawa in Japan, and they say;

Awakening from magical stasis after a thousand years, the Dark Lord Leonis suddenly finds himself in the body of a ten-year-old boy! He quickly meets Riselia, a girl confronting the Voids, creatures that have nearly exterminated humanity. Determined to uncover the mysteries of this strange new era, Leonis enrolls in Excalibur Academy, a school that trains students to fight back against these enigmatic monsters. Could the Voids hold some connection to Leonis’s past?

Amazon has light novels for sale, or some of them, in Kindle and paperback, and I can’t say the covers are all that reassuring.