Around Thirty Majutsushi no Yuruui Harem Life: Black Shachiku ga Isekai de Jiyuu Kimama ni Yuukyuu Shouka

Kyouhei regains memories of his past life as a powerful mage in another world. While his magic is useless in modern Japan, he decides to return to that fantasy world and live freely as an adventurer under the new name Tyler. Things seem easygoing—until the leader of the top-ranked party, Elza of the Valkyries, confronts him with a mysterious line: "I know your secret."

(Source: Futabasha, translated)