https://www.corocoro.jp/title/1084 https://ciao.shogakukan.co.jp/comics/title/00814

In 1989, in Kanto at the end of the Showa era, legendary biker gang leader Goro Takakura fulfills his dream of conquering all of Kanto. On that very day, however, he’s caught in a traffic accident brought about by his comrades’ betrayal. By some mysterious power, the place he ends up is… Tokyo in the 21st century. In the 17 years since he vanished, his only family—his younger sister—has gone missing, and the comrades who once burned with a sense of justice are now making their living in the underworld. To put everything back the way it should be, Goro Takakura once more raises his fists, this time together with new allies.

Satsuki, the daughter of a former comrade, appears before Renji, who was leading a slovenly daily life. "Teach me how to conquer Dungeons." Currently, Dungeon streaming has become a trend among young people. High school girl Satsuki is no exception. Renji was once the strongest conqueror in the country, but ten years have passed, and he has been completely forgotten by society, now just a dull old man. Dungeons, after all this time―― While thinking so, he couldn't just brush her off and ended up diving into a Dungeon... The dull old man who was only strong ten years ago is actually the strongest even today! A suddenly appeared out-of-date dull old man stirs up a storm in the Dungeon streaming world!!