The main protagonist of the story is a regular joe who reincarnates as an elf. He proceeds to become an incredibly adept mage after a century of training and sets off on a journey.
and was originally titled Tensei Elf ni Yoru 900-nen no Yukyu Musoki. A light novel titled Tensei Elf ni Yoru 900-nen no Yukyu Slow Life ~Zenzokusei Maho to Kodai Maho o 100-nen de Kiwameta node, Nokori no Jinsei wa Kikaru ni Okashimasu~ was released in 2022 under Starts Publishing’s Grast Novels label with illustrations by Hatori Kyoka. The manga version is drawn by kanco.
Sources: Press release, @comicgrast